140 Transactions of the [51fri[ Efffll 



right correction," as seen when the object is out of focus, illustrated 

 by coloured sketches ; experiments on the effects of varnish ; proof 

 that a compound lens has more effect on spherical and chromatic 

 aberration when placed beliind in a combination than when in 

 front, &c. 



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

 effects of glasses made by his own hands. These are referred to 

 in a letter to Sir John Herschel, of which he preserved a copy, 

 together with Sir John Herschel's reply. The letter is dated 

 London, 24th of 2nd month (Feb.), 1831. In it the following 

 passages occur : — " Finding, however, that W. Tulley was too busy 

 to pursue for me the experiments I wished for ascertaining how 

 compound object-glasses could be combined to the greatest advan- 

 tage, 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 36 degrees." " Long illness 



among my children afterwards 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 

 combination 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 occm-s the ingenious idea of regarding the triple 

 with the middle of flint glass as divided by an imaginary 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 himseff was " a construction fitted to obtain the 

 largest pencil with good front space and without coma ; " and after 

 describing the mode by which this was arrived at, he says, " This 

 combination proves most satisfactorily the advantage of keeping the 

 angles of the rays at all the different curves moderate, the vision 

 being singularly definite and easy Indeed, taking all toge- 

 ther, I think I have met with nothing to equal it — the distance 

 of the front glass from the object being O'll full." 



Having now completely satisfied himself of the applicabihty of 



