Paper on Aplanatic Object-Glasses. 295 



It is very true that the achromatic glasses composed of a 

 flint-glass, and a crown-glass, or double object-glasses, give 

 rays well corrected only for the central pencils, and confuse 

 the oblique ones ; but their advantages in clearness and in fa- 

 cility of construction, make them generally preferable to triple 

 ones. 



If the object-glass of Mr Lister has its concave tarnished, 

 the fault is to be attributed to the flint-glass of Guinand ; and 

 as to the appearance of dust visible in the substance which 

 unites the two glasses, we can assure you that it is almost im- 

 possible to avoid that very slight inconvenience in any object- 

 glass, whether cemented or not. The advantage of the glued 

 glasses is, that, if they include a little dust, they do not retain 

 any moisture between the surfaces which are in contact. 



The object-glasses of ten and fourteen lines must necessarily 

 be ineffective upon test objects, considering their focal distances. 



It is probable that some of our first object-glasses have not 

 been perfectly adjusted ; but this is also an imperfection which 

 it is not possible to avoid, rigorously speaking, in the construc- 

 tion of these minute glasses. For the rest, the approbation you 

 concede to our lenses in these words, " nothing can surpass 

 the beautiful simplicity of the curves of M. Chevalier," is doing 

 us a great honour. (See remark AA.) 



Euler composed his object-glasses of two bi-convex lenses of 

 crown-glass, and a bi-concave of flint, to form a triple object- 

 glass of six lines focal distance, (see the following extract from 

 the work of Euler,) with a large aperture ; and it is because our 

 microscope is achromatic, (though the lenses are differently dis- 

 posed,) that it is established on the principles of Euler. 



You find our quadruple combination of two double object- 

 glasses perfect ; but now we combine together three lenses of 

 short focal distances to form sextuple object-glasses. This com- 

 bination is scarcely possible except with our lenses, which have 

 the indispensable requisite for the perfection of microscopes, of 

 being very thin, and of a very small diameter, which conditions 

 cannot be easily fulfilled in tripic glasses. 



Our object-glasses are glued together with a mixture of tears 

 of mastic and turpentine, and that process ought to appear as 

 good in theory as it is in practice, the surfaces in contact hav. 



