The First English Achromatic Objective. By E. M. Nelson. 17 



B 



Fig. 15. 



The Lister Achromatic Combination (1827). 



In 1827 Mr. J. J. Lister put one of Chevalier's doublets as a 

 front, Tulley's triple being the back lens ; the combination was of ■ 52 

 focus, • 53 N. A., and 27 ' G O.I. This constituted the finest Microscope 

 objective that had up to that time been produced. (Fig. 15, A and B.) 

 It was, strictly speaking, the first really success- 

 ful scientific Microscope objective. Attempts 

 have been made to belittle Mr. Lister's work 

 in connection with the evolution of the Micro- 

 scope objective ; * in answer, allow me to 



present to you what is probably the true his 

 tory of the early Microscope objective. 



Mr. Lister, who had interested himself in 

 the Microscope from boyhood, began to work 

 at the objective in 1825, and it was the 

 possession of a -fa and a j 2 tT of Tulley's that 

 directed his attention to this subject. He 

 rightly judged that these objectives were too 

 thick, and; he persuaded Mr. W. Tulley to make for him a ^ much 

 thinner in proportion to its focal length. (The reason one hears 

 and reads so much about " thickness " in old books is that when //, = |, 



y 2 



the thickness is equal to -~, and this is the factor in one limb of the 



mathematical expression for spherical aberration ; so, by reducing the 

 thickness the spherical aberration is diminished.) Lister found that 

 the performance of this new lens was very nearly equal to Tulley's 

 best t 2 q ; he noted that whereas the ^ was free from " coma " the j% 

 and yTj were not free from it, except in a small part in the centre of the 

 field. The testing of a lens on a mercury globule and the observation 

 of " coma " at this early date indicates not only an important advance 

 but also a thorough acquaintance with the subject. 



In December 1826 Lister examined a set of four Chevalier-Euler 

 objectives, and found the apertures were much stopped down; he 

 improved them by opening out the stop. Afterwards he combined a 

 Chevalier front with a Tulley back, which made what may be called 

 the first scientific Microscope objective. Fig. 15 shows this combination. 

 In 1830 his paper on the twoaplanatic foci was read before the Royal 

 Society ; and towards the end of that year he took up lens-grinding 

 because Tulley was too busy to continue the experiments. In 1837 

 he began to coach Andrew Ross, and drafted for him a triple fronted I . 

 It is therefore manifest that Andrew Ross owed his success in a great 

 measure to improvements which Mr. Lister originated. In 1840 he 

 coached James Smith, who made 1-in., i-in. and £-in. objectives 

 on his formula. With regard to Powell, his older objectives show no 



* Society of Arts, Cantor Lecture, Dec. 21, 18S5. pp. 94, 05. 

 Feb. 10th, 1902 



