20 Object-glasses and their Definition. 



single lenses in front, they would emerge more nearly parallel, and 

 pass tliroiigh the middle at a more favourable angle, which could 

 therefore be made with a longer focus and radius. But the best 

 remedy would be an improved quality of glass. The flint with a 

 less refractive and higher dispersive power, and the crown vice versa, 

 I believe it possible that such a glass could be made. Of the triple 

 back little need be said, as the bending of the rays is here easy : 

 before this was introduced the old form had the defects to which I 

 now call attention in the present middle, and the marginal rays were 

 imperfect and limited the aperture. 



Speaking of recent improvements, Dr. Pigott alludes to an 

 object-glass made on the Lister principle as being " old-fashioned," 

 from which I infer that he considers them quite obsolete ; but as 

 long as the adjustment between the lenses exists for the purpose of 

 obtaining a correct aplanatic focus, so must all object-glasses remain 

 on the Lister principle, and it is the absolute correction of this focus 

 for chromatic and spherical errors upon which their perfection must 

 depend.* 



A great advance has been recently effected in the highest powers, 

 which are now made with a single front ; this after remaining for 

 years unnoticed has triumphed at last. This formula also affords 

 peculiar facilities for the addition of an immersion front, as I shall 

 presently show, and to which its success is mainly due. 



I will now make a few remarks concerning the Podura. In 

 the November number of the Journal, Dr. Pigott has quoted and 

 italicized some sentences from my communications, and placed them 

 so as to make it appear that I contradict myself, and have been un- 

 decided in the structure. These sentences, as my papers will show, 

 referred to the difiiculty of proving the spines to be projections by 



* I may here offer a passing tribute to the memory of tlie late Joseph Jack- 

 son Lister, to whose researches the present advance of the microscope object-glass 

 is entirely due, for in the perfection of aplanatic foci, caused by difference of cor- 

 rection, and corresponding distance between the lenses, he enunciated a fact as 

 definite as Newton's law of gravity. Some inclination has been shown to question 

 this ; but it must be remembered that in order to carry out his ideas, he had to 

 avail liimself of tlie skilled workmanship of the then three rival makers, who, 

 doubtless individually, eftected many improvements in detail in the particular 

 power that they engaged to construct, and each justly considered that he had 

 some right to a monopoly in the special objective that his workmanship had made 

 perfect. This rendered Mr. Lister's position rather a delicate one, and his remark- 

 ably amiable and kindly disposition forbade him from coming forward to the injury 

 of others, and his retiring manner shrunk from anything approaching to public 

 display ; hence he has been unjustly suspected of reserve. I visited him at his 

 residence at Upton on two occasions, and on each spent several interesting hours 

 in going through all his plans and experiments, which were brought forward in 

 the most candid and confiding manner. Every proposed construction had been 

 worked out, with all the rays accurately traced through, in large and neatly-cou- 

 structed diagrams, before they were carried out practically ; and I should be glad 

 to see these very comprehensive plans in the archives of the lioyal Microscopical 

 Society, as I know for a fact that ^^ths, ^ths, and ^ths, were made therefrom, with 

 no other al teration than a slight variation in the quality of the glass required. 



