OBITUARY NOTICE OF JOSEPH JACKSON LISTER 551 



the discovery of the existence of two aplanatic foci in a double achromatic 

 object-glass. This has formed a basis for subsequent important improvements, 

 the object of which has always been to obtain sharpness and achromatism over 

 the field in the picture from a larger and larger pencil ; this being an essential 

 to obtaining higher and higher defining power. 



' After succeeding fairly in a trial combination with this view, I left the 

 subject for a while, hoping it would be pursued by opticians. But the glasse> 

 l)roduced by the makers continued to be on the first simple construction of 

 two or three plano-convex compound lenses till the beginning of 1837. At 

 that time I called on Andrew Ross regarding some object-glasses he had made 

 to a microscope for Richard Owen ; when he told me he had been long engaged 

 in unsuccessful trials for a new construction. And at his request I gave him 

 a projection for a |--inch objective of three compound lenses, the front on* 

 a triple, which he soon worked out successfull}', and it became the standard 

 for high power for many years. 



' For lower powers I suggested at the same time a double combination, and, 

 borrowing of him a lens from among his former failures, and applying it in 

 front of one of my own at home, obtained at once the performance required. 



' It was natural that A. Ross should regard these as trade secrets ; and 

 accordingly, in his article on the microscope in the Penny Cyclopaedia he does 

 not mention them, giving only the earlier construction of my article in the 

 Transactions. The same is given afterwards in the treatise which J. Ouekett 

 asked at the point of its publication to dedicate to me ! And I did not feel 

 required to disclose A. Ross's secrets. After a while, with his consent, I in- 

 structed James Smith, 1840, to execute the same construction for inch and 

 half-inch glasses. Even in 1843 it was with the understanding that he should 

 not go to deeper powers than |^-inch, and "Smith's quarters" were long in repute. 

 In these projections the endeavour was to keep the angle of pencil at each surface 

 of the glasses as moderate as was consistent with the other essentials ; and by 

 degrees the pencil admitted has been enlarged beyond my expectations. Some 

 variations too have been since made in the construction in wliich I have had 

 no part ; but for all, the principle of the two aplanatic foci has furnished the 

 clue.' 



I believe I am correct when I state that in foreign microscopes also, object- 

 glasses of high powers and fine performance are constructed on the same jirinciple. 

 And thus it seems not too much to say, as has been lately said by a Professor 

 in one of our Universities — the son of one who was fornierh' associated with 

 my father through a common love of science — that he was ' the pillar and source 

 of all the microscopy of the age '. 



