

Paper on Aplanatk. Object-Glasses. 221 



with the most achromatic glass, by a simple modification of 

 illumination, we can show the outlines of the object fringed, 

 with colours, but by a little change in the position of the re- 

 flecting mirror, this can be made to disappear entirely. (See Re- 

 mark E.) This i?-is or rainbow often proceeds from the eye- 

 glass. This is so true, that, with a catadioptric microscope, it 

 happens sometimes that the outlines of objects are coloured. 



I believe that when we compare the effect of two achromatic 

 lenses, they must be of the same focal distance, if you wish to 

 judge rightly of their merit, as to their freedom from disper- 

 sion and spherical aberration ; therefore, it would not be right 

 to compare (with reference to these qualities,) a lens of six 

 lines, or of an inch focal distance, with an achromatic system 

 representing a power of less than one line. The best object- 

 glass, in my opinion, is that which is the most penetrating, viz. 

 that which shows with most nicety the little details of test-ob- 

 jects, even if it gave a little tinge of colour, which it is almost 

 impossible to destroy without a great reduction of the aper- 

 ture ; but then what is the use of a perfectly achromatic 

 object-glass, if it has no power over the test-objects ? 



Certainly I do not deny the advantage of a glass of short 

 focal distance perfectly free from the aberration of sphericity 

 and refrangibility, as it would fulfil the conditions of the high- 

 est degree of perfection. I have seen achromatic lenses of all 

 manufacturers. More than a thousand of these glasses have 

 passed through my hands, and I have never found that ulti- 

 matum so much to be desired ; therefore, I still believe that 

 the most penetrating object-glass will be the most useful, and 

 that to which the preference must be given. As to other matters, 

 be pleased to let me know what you think of the system of len- 

 ses, belonging to Mr Cuthbert, which is composed of the len- 

 ses marked. 



of 12 lines focal distance, lens plano-convex. 



1 of 4 lines do. - - do. 

 '2 of 4 lines do. - - do. 

 .'} of 2 lines do. - meniscus. 



Three of these lenses are always employed together, wheth< i 

 vmi lake awaj the glass marked 3 fir that marked 0, and em 



