28 Transactions of the Society. 



lonsr ensacfed in unsuccessful trials for a new construction, and at 

 his request I gave him a projection for an \-m. objective of three 

 compound lenses, the front one a triple, which he soon worked out 

 successfully, and it became the standard form for high-power for 

 many years. 



" For lower powers I suggested at the same time a double com- 

 bination, and borrowing of him a lens from among his former 

 failures, and applying it in front of one of my own at home, ob- 

 tained at once the performance required. 



" It was natural that A. Eoss should regard these as trade 

 secrets, and accordingly in his article on the Microscope in the Penny 

 Cyclopedia he does not mention them, giving only the earlier con- 

 struction of my article in the Transactions. The same is given 

 afterwards in the treatise which J. Quekett asked, at the point of 

 its publication, to dedicate to me ! and I did not feel required to 

 disclose A. Eoss's secret. After a while, with his consent, I in- 

 structed Mr. Smith (1840) to execute the same construction for 

 inch and J-in. glasses; even in 1843 it was with the understand- 

 ing that he should not go to deeper powers than J-in,, and 

 '' Smith's :^-in." were long in repute. In these projections the 

 endeavour was to keep the angle of the 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 which I have had no part — but for all, the principle 

 of the two aplanatic foci has furnished the clue." 



[Lister's great son would also appear to have felt that greater credit 

 should be given to his father, as witness this letter :] 



" 9 Stanhope Street 



Ibth of 10 mo., i850. 



*' My Dear Father, 



I have just received and read Mamma's note, from which I 

 am glad to learn that you purpose going to Hitchin on 5th day. 

 As I shall probably not see thee on 7th day, I will just tell thee, if 

 I can, something of the character of Potter's introductory lecture, 

 which I attended this afternoon. He began by speaking of the 

 necessity of the observation of Nature, and of how theories must 

 always be made to give way to facts, etc. He then proceeded to 

 give a sketch of the principles of optical instruments in general, 

 and of the sources of imperfection in glasses made with spherical 

 surfaces, viz. aberration, colour, and, in case of the Microscope 

 coma. He also gave a short account of the discoveries of 

 Newton, Dollond, and Huygens, by which the telescope had been 

 successively improved. He explained then the great difference 

 between the telescope and Microscope, in the infinitely small angle 



