Iconantidiptic telescope (1821)
Achromatic telescope without lenses, and with a single refractive medium (1821)
Prism reflecting sector (1822)
Instruments for measuring the parallel situations at the horizon (1837)
New camera lucida and positive achromatic eyepiece (1839)
Telescope which can be used on all the geodesic instruments (1841)
New catadioptric microscope (1842)
Direct vision prism (1857-1860)
“The iconantidiptic telescope by Amici offers many useful applications; it produces two images of the same object, one formed by the rays that cross one half of the objective and go directly to the focus of the eyepiece, the other by the rays which, after having crossed the other half of the objective, before arriving to the focus of the eyepiece, are reflected and refracted by an isosceles prism situated inside the tube of the telescope with its unequal side parallel to the optical axis of the telescope itself. Thanks to this ingenious construction the two images of a star appear to move in opposite directions, and they overlap when the star, in its movement, places itself precisely onto the lengthened optical axis of the telescope. One therefore can determine the precise position of stars in a given instant, without needing filar micrometers, that is, one can resolve the fundamental problem of practical astronomy. And this can be done without illuminating the field of the telescope during nocturnal observations, which is very advantageous when the stars are dim, as are many that modern astronomy especially considers. Amici constructed his Iconantidiptic Meridian along the same principle.” (G. B. Donati, Elogio del Prof. Gio. Battista Amici, Cellini, Firenze 1865, p. 14).
Amici described this telescope in his paper of 1821 Sopra un cannocchiale iconantidiptico (About an iconantidiptic telescope), published in “Memorie di Matematica e di Fisica della Società Italiana delle Scienze”, Tomo XIX, p. 113-120 (download pdf).
Achromatic telescope without lenses, and with a single refractive medium (1821)
“Amici found a way to construct an achromatic telescope without lenses, with only one refracting medium. He was led to this construction by the discovery that he made of a property of refracted light. This property, even if (as he demonstrated) it is an immediate consequence of the theory of refraction, had never been observed by anyone before him” (G. B. Donati, Elogio, ibid., p.14).
In equal deviations of the spectrum produced by unequal deviations of rays onto the two sides of a prism, the coloured spaces are greater when the deviation on the posterior side is greater compared to the anterior, and this only in the case in which the incident ray has not been first refracted in any way. Because if the incident ray has already been refracted by a prism, in equal deviations of the spectrum produced by unequal deviation on the two sides of the latter prism the coloured spaces are lesser when the deviation on the posterior side is greater compared to the anterior. (G.B. Amici, Sulla costruzione di un cannocchiale acromatico...).
“Using the law he had discovered as a basis, Amici constructed a telescope without lenses, composed only of four plane sided prisms, all of the same quality of glass, with a magnification of about four times, and perfectly achromatic. And he therefore proved that even without using two differently refracting or dispersive substances it would have been possible, as early as Newton - who was convinced of the opposite - to have constructed achromatic telescopes.” (G. B. Donati, Elogio, ibid., p. 15).
The telescope is described in the paper of 1821 Sulla costruzione di un cannocchiale acromatico senza lenti eseguito con un sol mezzo refringente (On the construction of an achromatic telescope without lenses, and with a single refractive medium), published in the “Memorie di Matematica e di Fisica della Società Italiana delle Scienze”, Tomo XIX, p. 121-137 (download pdf).
Prism Reflecting Sector (1822)
In September of 1820, the Baron Franz Xaver von Zach was visiting Amici’s workshop in Modena. From among the many instruments there his attention was particularly attracted to a combination of two prisms of isosceles glass, through the movement of which the angular distance of two faraway objects could be measured. Amici’s intention had been that of providing topographers and navigators with a small, easy-to-use and easily rectified instrument with which they could measure even the angles beyond 180 degrees (posterior observation) with a high degree of precision.
Encouraged by his guest, Amici improved the instrument and he described it in a letter to the Baron dated 3 July 1822. Von Zach published it with the title Sur un nouvel instrument de réflexion in the “Correspondance astronomique”, Sixième Volume, n. VI, 1822,
p. 554-560 (download pdf). The appreciation which this met led Amici to create an entire Prism reflecting circle some years after.
Zach, who had tried out the instrument in May of the same year in Genoa, together with the Swiss astronomer Johann Kaspar Horner (1774-1834), spoke very flatteringly of it in a long note which he published after the Lettre from Amici (download pdf).
Instruments for measuring the parallel situations at the horizon (1837)
Besides the floating level fitted with a Galilean telescope (see Level), Amici created other instruments for determining the horizontal plane and he described them in the paper Sopra alcuni Istrumenti che servono a conoscere le situazioni parallele all’Orizzonte (About instruments used to identify the parallel situations at the horizon), read at the Accademia dei Georgofili on 5 March 1837, and published in the “Atti” of the Academy, Volume XV-1837, p. 129-136 (download pdf ).
The first is an improvement introduced in the level that land-surveyors used to place the Praetorian tables horizontally, consisting of “a sheet or flat strip of metal over which a tube of glass containing wine spirit and the bubble of air is securely fastened”.
Another level he imagined
consists of a single glass isosceles prism. By placing its main face on the surface of the mercury, the prism balances itself and a horizontal ray coming from an object, after being refracted and reflected in the prism, exits parallel to the first direction. Now by applying the eye to the emerging ray, and at the same time looking outside with a part of the pupil, objects are seen both in their natural position and upside down: and it is clear that all the coincident points of the straight and upside down images will be on the horizontal plane that passes through the surface of the mercury. An ounce of this metal and a little box of the diameter of a coin which contains the mercury as well as the prism would form the entire instrument.
But considering that large operations “require machines of a higher order than the pocket-sized little level just outlined”, he proposed a larger level aimed at satisfying the most delicate research projects of Geodesy. This level had three parts: the Base, the Regulator and the Telescope, “to which the axis of rotation in azimuth and the tube with the air bubble are solidly connected”.
Explanation of Figures 1, 2, 3 which are the level, the regulator and the base.
A, B. Brass pipe which contains the glass tube, which is divided with a diamond in inches and lines on the top to indicate the ends of the bubble and to determine the angular deviations, which in the present level are three minute seconds for every line of the Paris foot.
C. Screws which allow lateral movement to tube A, B.
D. Screws which move the same tube up to down and vice versa.
E, F. Objective lens and eyepiece of the telescope which magnifies 30 times. The double eyepiece F of a Ramsden form can be substituted by a quadruple eyepiece which shows objects vertically.
G. Tube which holds the crossed threads, and besides the longitudinal movement to remove the parallax also turns around the optical axis with two button screws H which press against a protruding thin sheet and in this way one of the threads becomes horizontal.
L. Azimuth rotating axis.
M. Its correcting screws.
N. Empty cone in which the aforementioned rotating axis enters.
PQ, RS, TV. Profile of three thin sheets which form a cross when viewed on a plane. The one in the middle which is pinned at the lowest one with the ends of its U-arms receives a swinging movement from the screw Y which the spring X acts upon. The other cross at the higher point with the action of screw Z and of the corresponding opposed spring, takes a similar movement of swinging around the poles P, Q, in a right angle to the first. In this way with the use of screws Y, Z alone the axis of rotation in Azimuth is promptly placed in the vertical.
K. Brass screw in the centre of the tripod onto which the Regulator is screwed.
h. Screws which fasten the triangular legs g to the table above.
On 14 October 1839 at the First Meeting of Italian Scientists in Pisa Amici also read a presentation about two optical devices recently invented by him: a new camera lucida and a positive achromatic eyepiece (cf. Atti della Prima Riunione degli Scienziati Italiani tenuta in Pisa nell’Ottobre del 1839< Acts of the First Meeting of Italian Scientists, held in Pisa in October of 1839 > Nistri: Pisa 1840, p. 49-50). The main piece of his new camera lucida
is an isosceles triangular prism of crystal whose unequal face is larger than the others and amalgamated as a mirror. Its benefits are the very clear images and the notable extension in the field of vision. The Author then presented Wollaston’s Camera Lucida and pointed out how the advantages mentioned make his own much preferable to Wollaston’s.
He then continued speaking about his other invention, which consists in an eyepiece in the form of those which are called positive, which he was able to render achromatic with the use of two pieces of glass of different dispersions while maintaining its ability to take in a visual angle almost double that of the common eyepieces. The disappearance of the colours is obtained with an excess of chromatism of the lens nearest the eye such as to compensate the inverse dispersion of the other lens which remains on the side of the objective.
At the Third Meeting of Italian Scientists in Florence on 16 September 1841 Amici exhibited a telescope whose use could be extended to all geodesic instruments and explained how it could be used to more easily rectify the level (cf. Atti della Terza Riunione degli Scienziati Italiani tenuta in Firenze nel Settembre del 1841< Acts of the Third Meeting of Italian Scientists, held in Florence in September of 1841 >, Galileiana, Florence 1841, p. 200-201).
To understand how this happens it is useful to notice that in the ordinary sight, the so called pinnule-sight, the optical axis does not suffer deviation, and by moving the eye from one end to the other of the sight rule one can aim at two objects, and so with a simple inversion one can check the horizontal position of the visual. But it is also useful to note that through the sight rule, which does not magnify, one cannot aim at a point of an object with the same precision which can be obtained using a telescope. For this reason Prof. Amici thought to place two equal objectives at the end of a cylindrical tube, the distance of which equals the sum of the focal lengths, and which have a reticule in the common focus. In this way, this tube, or telescope (which does not magnify at all) can be used in the same way as a sight rule, and it is possible to look from one part or the other of it without changing the collimating line. To obtain the desired magnification, the inventor later added a small astronomical telescope to the two ends of the tube, so that all the system forms a terrestrial telescope composed by four lenses.
On 26 September 1842, at the Fourth Meeting of Italian Scientists in Padua, Amici presented his invention, a new catadioptric microscope (cf. Atti della Quarta Riunione degli Scienziati Italiani tenuta in Padova nel Settembre del 1842 < Acts of the Fourth Meeting of Italian Scientists, held in Padua in September of 1842 >, Seminario, Padua 1843, p. 448-449).
The catoptric lens of this instrument is composed of a round disk of glass of about a half inch in diameter and of even lesser thickness. Having first established the distances at which one wishes to place the object to be magnified and the focus of the eyepiece, the inventor uses these elements to determine the curvature to give to the glass disk to transform it in the following way in a very small, upside down, Cassegrain telescope. That is, he works one of the faces of this disk into a spherical form like a convex lens, and he makes the other face concave only in a small, central portion.
He then applies a sheet of tin to the two (convex and concave) surfaces, obviously obtaining two mirrors, one opposite the other, as in the abovementioned telescope.
Now, in order to prepare them for microscopic magnification, the rays must arrive from the object to the convex mirror without refracting as they enter the glass, and then reflected from the convex to the concave, and from this returned to the eyepiece, they are not refracted even as they exit the glass. To create these conditions, Amici formed a spherical indentation with the radius of curvature equal to the distance of the object from the mirror in the centre of the larger mirror, and this indentation is left without tin. On the other end, around the convex mirror, he gives the glass surface a radius of curvature equal to the distance of the objective from the place of the image in the eyepiece. In this way the rays do not refract in any way, but only reflect, and so are not decomposed, and the microscope seems to be formed by metal mirrors alone. In fact, he says that it is even more effective because less light is lost in the reflection by glass mirrors, and they can be constructed in very small dimensions without needing any support to keep them centred; the support, with its opacity, would take away part of the rays which form the image.
Between 1857 and 1860 Amici proposed an instrument for the observations of the striae of the stellar spectra to his assistant Giovanni Battista Donati.
“The difficulties which have to be overcome in this type of research”, wrote Donati in the paper Intorno alle strie degli spettri stellari < On the lines in stellar spectra > (“Il Nuovo Cimento”, vol. XV, 1862, p. 296-302), are principally two”:
In the first place a great number of rays must converge in the formation of the stellar spectra because otherwise (given the very weak light of the stars) these spectra would not have enough light to be seen and studied. In the second place there must be some means of giving these spectra a sufficient height, because if they were simply produced by the direct rays of the stars (which appear like optic points to us) they would have such a small height that it would be difficult and nearly impossible to recognize them and to observe their striae. I will not recount the entire history of all the instruments I began planning as early as 1857 in order to overcome these difficulties, because having informed Prof. G. B. Amici of my ideas, he suggested a device that would be more suitable for the purpose than any other and which I used to make the observations described in this Paper.
In order to have the greatest amount of light possible, the large burning lens of 41 cm in diameter and 158 cm of focal distance, left to the Grand Duke Cosimo dei Medici by Benedetto Bergans of Dresden on his way through Tuscany after 1690, which was kept in the Tribune of Galileo in the royal Museum in Florence was used. Near the focal point of the lens a narrow slit was placed where a great beam of light gathered and could easily be examined by the prism. Amici suggested “positioning between the prism and the slit a small lens, the focus of which is coincident with the slit. So the rays leaving the slit pass through the small lens and then meet the prism, all in parallel directions [...] I mounted this lens parallactically over a mobile stand, as can be seen in Table I, Figure 1. [...] Figure 2 shows (on a much larger scale than Fig. 1) the other part of the instrument which contains the prism, the slit and the small telescope with which the striae can be observed».
in order to make observations easier (making it easier to point to the stars) has now constructed a prism which offers very great dispersion without deviating the axis of vision: this prism is composed of three prisms, two of which are of crown-glass and between them there is a third of lead boron-silicate. If one looks directly at a slit or a luminous line with this prism, the light is seen as decomposed, and the spectrum presents the same striae which appear through a simple prism of flint glass.
In the Elogio del Prof. Gio. Battista Amici he then remembered the article Note sur trois spectroscopes présentés (“Comptes Rendus de l’Académie des Sciences”, vol. LV-1862, p. 576-578), in which the French astronomer Jules Janssen described, among others, the compound prism of Hoffmann:
Le second instrument est un spectroscope de poche, il est également à vision directe, et forme une très-petite lunette qui peut se replier sur elle-même. Le redressement du faisceau est obtenu au moyen d’un prisme composé construit sur le principe de M. Amici, qui est formé, comme on sait, d’un prisme central en flint très-dispersif accolé à deux prismes de crown à sommets opposés, et qui redressent le faisceau.
In 1909 P. Salet wrote, about direct vision prisms, that “Ce système de spectroscope est un de ceux qui ont été le plus employé en Astronomie. Janssen l’à présenté à l’Académie en 1862 et s’en est servi à Rome avec Secchi pour faire des observations de spectres d’étoiles. Ce spectroscope est formé essentiellement d’un prisme d’Amici, c’est-à-dire comprenant deux prismes de crown et un de flint collés avec du baume de Canada et disposés dans l’ordre représenté par la figure 4. L’effet de ce prisme est, en quelque sorte, l’opposé de celui du prisme achromatique, c’est-à-dire qu’il ne dévie pas la lumière des rayons moyens, bien qu’il disperse encore, parce que la dispersion du flint est de beaucoup prédominante. L’opticien Hoffmann porta à cinq le nombre des prismes: deux en flint pesant et trois en crown” (P. Salet, Spectroscopie astronomique, Doin, Paris 1909, p. 41-42).