achromatic compound Microscope. 171 



the glasses have been combined together, and that of the short- 

 est 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 ef- 

 fects of these will be again adverted to. 



The eminent professor of Modena, besides inventing his 

 well-known reflecting 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 re- 

 sume it in 1824, from reading the report already mentioned 

 on the microscope of Selligue : from this Amici took the thought 

 of combining his object-glasses, and pursued it with great suc- 

 cess, 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 1827, 

 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 aper- 

 ture,'" which considerably excels them. 



The glasses which have been enumerated possess very differ- 

 ent 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 imajie which characterize a fine achromatic. 



When the rays received by the most perfect object-glass 

 from any indefinitely 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 "harply defined 

 point if moderately magnified, is really a spot or small circle, 

 and will show as such if the microscope is sufficiently over- 

 charged with power in t.lie eye-glass. These circles bear a con- 

 siderable analogy to the spurious disks of stars; and like them 



