OBITUARY NOTICE OF JOSEPH JACKSON LISTER 54^ 



* Memoranda on object-glasses made for experiment, December 1829 to Mav 

 1830 '. These include a great number of interesting observations, such as 

 trials of lenses of different forms ; descriptions of the * colours of over, undei 

 and right correction ', as seen when the object is out of focus, illustrated b\' 

 coloured sketches ; experiments on the effects of varnish ; proof that a compound 

 lens has more effect on spherical and chromatic aberration when placed behin-^l 

 in a combination than when in front, &c. 



Then follow a set of notes of peculiar interest, describing the effects ui 

 glasses made by his own hands. These arc referred to in a letter to Sir John 

 Herschel, of which he preserved a copy, together with Sir John Herschel's repl\ 

 The letter is dated London, 24th of 2nd month (February), 1831. In it the 

 following passages occur : ' Finding, however, that W. Tulle\' was too busy 

 to pursue for me the experiments I wished for ascertaining how compound 

 object-glasses could be combined to the greatest advantage, I determined in 

 November last to make a trial myself. The result was, I acknowledge, beyond 

 my expectations ; for without having ever before cut brass or ground more 

 than a single surface of a piece of glass, I managed to make the tools and to 

 manufacture a combination of three double object-glasses, without spoiling 

 a lens or altering a curve, which fulfilled all the conditions I had proposed for 

 a pencil of thirty-six degrees. . . . Long illness among my children after- 

 wards absorbed all my leisure till about three weeks ago, when I made a second 

 and more complicated trial, projected for obtaining the same effect with a much 

 larger pencil. This is just finished, but not without altering one of the original 

 curves ; and its plan might be improved if I could spare time to make another 

 set. Still, I flatter myself these attempts would interest thee, as showing how 

 easily the principle I mastered may enable an utter novice in glass-working to 

 produce vision which I have not yet seen exceeded.' In the second of these 

 trials he deviated from the plano-convex form of the lenses, employing a com- 

 bination of three, of which the front was a double meniscus, the middle a triple, 

 and the back one a double plano-convex. The reasons for preferring these 

 forms are given in full detail in his notes, among which occurs the ingenious 

 idea of regarding the triple with the middle of flint glass as divided b\- an imagi- 

 nary line through the flint into two double achromatic glasses, each of which 

 may be considered separately as having two aplanatic foci. The object he 

 proposed to himself was ' a construction fitted to obtain the largest pencil with 

 good front space and without coma ' ; and after describing the mode b\- which 

 this was arrived at, he says, ' This combination proves most satisfactorilx the 

 advantage of keej^ng the angles of tlie rays at all the different rur\es moder- 

 ate, the vision being singularly definite and easy. . . . Indeed, taking all 



