146 MICEOSCOPIC VISION. 



hard for iion- mathematical microscopists to think so, — to 

 believe that so much mysterious mathematics can have 

 nothing in it, especially after their nerves have been shaken 

 by meeting such words as Pneumo-spherical and Hydro- 

 spherical, Aberrameters, Kratometers, Refractometers, 

 Eidola, and all the rest. But so it is. The whole of this 

 ' original ' learning from beginning to end, the picture of 

 the bent oar not excepted, is simply copied out of the first 

 pages of the little books for beginners, the primers and horn 

 books of optics. Examples are given in the primers to be 

 worked out, and Dr. Pigott works out other examples with 

 the numbers changed, — that is all. The * original ' part 

 is that he works them with infinitely more trouble than 

 is needed." 



Enough has been said to put you in a position to judge 

 of the state of microscopic optics at tha-t time (1870-71). 

 But before passing on, it may be as well to point out that 

 it is more than possible that this hint of Dr. Pigott's of 

 turpentine immersion (homogeneous as regards refractive 

 index, but irrational as regards dispersion) suggested to 

 R. B. Tolles the balsam immersion objective which he had 

 in 1874 actually constructed. Tolles being a practical 

 optician, must have known that turpentine had far too 

 high a dispersive power to be of any use for immersion 

 purposes. 



The correct principles underlying the measurement of 

 apertures had been investigated and published by Tolles,^ 

 w^ho later, in March, 1873,^ was the first to describe an 

 apertometer, consisting of a semi-circular disc with slides, 

 much the same as that now known as Abbe's Apertometer. 

 Apertures in those daj'S w^ere measured by angles in glass 



1 M. M. J., vol. vii. p. 117, fig. 1 (1872). 



2 M. M. J., vol. ix. p. 213, PI. XV. (March, 1873). 



