Memoir of the late Mr iniliam Black ie. 97 



his moulds and tools were made by his own hands, and were 

 so insignificant in appearance, that they were seldom recog- 

 nised even by persons conversant with the like pursuit. Some 

 of his processes in lens-grinding and polishing were entirely 

 of his invention ; and, indeed, in almost every step of his pro- 

 gress, he strove to master difficulties by the exertion of his 

 own ingenuity, rather than have recourse to the advice or 

 assistance of others. To Mr George Saunderson, however, 

 he acknowledged himself indebted for his having described 

 to him a simple method of executing the spherical surfaces of 

 garnet lenses. 



By unremitting exertions, he produced numerous micro- 

 scopic lenses of excellent workmanship and high power in the 

 course of four years. He was particularly successful in mak- 

 ing the bird's eye lenses, or grooved spheres, now so much 

 used as pocket microscopes. He also brought to great per- 

 fection achromatic object-lenses for the compound microscope. 

 These have been pronounced by competent judges to be en- 

 tirely free from spherical aberration, and to aiford an ex- 

 tended field of vision. The following testimony to the sur- 

 passing merits of one of these lenses is extracted from the 

 article " Microscope," in the last edition of the Encyclopaedia 

 Britannica : — " Mr Pritchard remarks, that when the lens 

 next the object is a jewel, the performance of the doublet is 

 improved ; but that he has not observed any advantage when 

 both lenses are gems. This must be a mistake ; for lenses 

 of any gem that are superior to glass ones when acting singly, 

 must, if suitably combined, be superior also when united. In 

 proof of this we have a garnet doublet before us, executed by 

 Mr Blackie, the performance of which is quite remarkable. 

 The lenses are made of Elie garnets, and their convex sides 

 are placed towards each other. The radius of the smallest 

 lens near the object is l-70th of an inch, and that of the 

 other l-20th of an inch. Its magnifying power is very high, 

 exceeding greatly that of the semi-jewel doublet made by Mr 

 Pritchard, with a sapphire lens l-60th of an inch focus, com- 

 bined with a glass lens 1-lOth of an inch focus." 



After having so far distinguished himself by these beauti- 

 ful works, Blackie was encouraged to visit London in 1836, 



VOL. XXXVII. NO. LXXIII. JULY 1844. G 



