ACHROMATIC COMPOUND MICROSCOPES. 
189 
those artists, printed in 1829 , that it is only lately the glasses have been com- 
bined together, and that of the shortest focus added to the former series of 
four ; and also that they are intended to be used in the order of their focal 
lengths, the shorter towards the object. Each of these glasses singly admits 
but a small pencil of from 8 ° to 15 ° of light, and when so used, their defining 
power is necessarily not very great ; but their combinations have much more, 
and the different effects of these will be again adverted to. 
The eminent professor of Modena, besides inventing his well-known reflect- 
ing microscope, was engaged about the year 1815 with achromatic object- 
glasses ; but as they did not equal his reflector, he laid the work aside, till he 
was induced to resume it in 1824 , from reading the report already mentioned 
on the microscope of Selligue : from this Amici took the thought of com- 
bining his object-glasses, and pursued it with great success, making them 
double, with the curves of the two lenses of each planned for the place it is to 
occupy, and for obtaining a good image either with the back glass alone, or 
with a second or a third in front. 
He brought with him, when he visited London in 182 /, some glasses of this 
description of very fine performance ; and I have been informed by him that 
he has since executed a combination of “ 2.7 lines in focal length and 2.7 
lines in aperture,” which considerably excels them. 
The glasses which have been enumerated possess very different degrees of 
merit, chiefly dependent on the extent to which they are divested of chromatic 
and spherical aberration, and particularly, in connection with this, on the focal 
angle of their aperture ; for it has been well established, that a large pencil 
from the object is absolutely essential to that brilliancy and distinctness of 
image which characterize a fine achromatic. 
When the rays received by the most perfect object-glass from any indefi- 
nitely small bright portion of an object in the centre of its field are brought 
together at its conjugate focus, the image formed by them, though it appears a 
sharply defined point if moderately magnified, is really a spot or small circle, 
and will show as such if the microscope is sufficiently overcharged with power 
in the eye-glass. These circles bear a considerable analogy to the spurious 
disks of stars ; and like them they will be found to be much enlarged by 
diminishing the aperture of the object-glass. 
They are enlarged also, without contracting the aperture of the glass, by 
