Boijal Microscopical Society. 95 



tion," in which he describes a plan for getting from a single con- 

 denser two or more pencils of light at any angle to each other — a 

 plan which commended itself much to those engaged in determining 

 the markings of diatoms, " the principle sought to he carried out 

 being to throw the axis of the pencil of illuminating rays in a 

 direction at right angles to the line to be resolved," the great 

 advantage of the plan being " simple, cheap, and easy of adjust- 

 ment ; " by means of it the dots on the N. angulatum are easily 

 resolved with a half-inch object-glass. This condenser he sub- 

 sequently improved by the addition first of one and then of another 

 lens to the original Hemisphere, continuing his original plan for 

 applying stops. 



The subject of illumination in microscopic research was always 

 a favourite study with him, and he has often been heard to say 

 that the microscope was nothing without it, and it was from 

 having this subject so constantly before him that he was led to 

 the discovery of the value of a parallel beam of light in deter- 

 mining the true structure of the markings in diatoms and other 

 mmute objects. In June, 1869, he read a paper before the 

 Society, descriptive of the prism which now goes by his name, 

 the value of which cannot be over-estimated ; for this inexpensive, 

 simple appliance gives results which are not surpassed by any of 

 the numerous contrivances of Wollaston, Brewster, Amici, Gillet, 

 Kingsley, &c., or even his own favourite kettledrum, with flute-key 

 adjustment, and it is interesting to note his own version of the 

 step which led to its discovery and value. He says, " I very fortu- 

 nately purchased of my friend, Mr. Powell, his little hemispherical 

 condenser for showing the transverse lines of that prince of puzzles, 

 the Amphipleiira i^ellucida. In using this lens I saw on reflection 

 that a small portion only was available. I therefore covered the 

 whole lens with tinfoil, and then cut out opposite apertures on the 

 plain and convex surface, so as to obtain, in point of fact, a small 

 prism illuminator. This answered the purpose well, and it imme- 

 diately occurred to me that the equilateral prism itself might be 

 substituted for the prism slice of Powell's lens. I tried it, and saw 

 at once the great advantages of the equilateral piism and its single 

 parallel pencil of light. There are, indeed, many facts and secrets 

 of structure which can be revealed by such a beam alone; light 

 virtually parallel can lead to no confusion, but the crossing and re- 

 crossing of an infinite number of rays produce such a multiform 

 shadow as to de-shadow or obliterate a true hght and shade por- 

 traiture, which is the essence of every picture, and the very soul of 

 every natural representation." 



Dr. Donkin, who is now engaged in editing the ' Natural History 

 of the British Diatomace?e,' bears testimony to the great value of 

 the prism in the examination of these interesting objects. 



