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VI. — Mr. ToUes " Experiments on Angular Aperture." 



By F. H. Wenham, Vice-President E.M.S. 



The above essay would not have required any special notice from 

 me, had not reference been made to my name. I consider that I 

 have already said enough to settle the question in minds familiar 

 with optical science, but there are others who might suppose, from 

 the evidence of these experiments, that I have assumed things 

 incorrectly, for there appears to be a lack of discrimination in this 

 subject that quite justifies the remark made by the Kev. S. L. 

 Brakey " as showing the incredibly low level at which the scientific 

 knowledge of optics exists amongst microscopists." 



Mr. ToUes' communication is set forth in an unassuming way, 

 that does not provoke any hard strictures, but it enters within the 

 hues of a controversy that has been long and hotly contested in 

 reference to a subject which it would seem remains yet to be 

 understood. 



As shown by the figure, Mr. Tolles fixes before the front lens 

 of an objective, a plano-convex lens nearly hemispherical, and then 

 measures the aperture. " On testing the angle, only 80^ (or at 

 most, less than 82") was obtainable. Were the plano-convex 

 removed, the angle indicated would be 170^ upwards. This was 

 verified carefully," There is a simple honesty and truth in this 

 experiment that is very pleasing. Half of 82^, or 41°, would be 

 about the angle of total reflexion from the flat surface of the 

 piano-lens, at which a veil of utter darkness would be thrown 

 between the object-glass and the hght, and none could pass beyond, 

 so that after this the plano-convex had nothing whatever to do 

 with apertures exceeding 82^. 



We next come to the immersion question. I must compliment 

 Mr. Tolles on the accuracy of his httle diagram. The hemi- 

 spherical lens is, I have no doubt, correct, and so is the front of the 

 objective, both for diameter and thickness, as it should be, coming 

 from a professional maker. Mr. Tolles says, " When the air in this 

 interspace (i. e. between the lenses) is replaced by water, the angle 

 becomes 100°, or a httle more (an after-note states it as 110). 

 ... It seems incontestable, at all events, that more than 82° of 

 angular pencil can traverse the balsam-mounted object, and be 

 transmitted by the immersion objective to the eye of the observer." 



Though I saw at a glance why this assertion must be incorrect, 

 and could explain the reason in a few words, yet as Mr. Tolles has 

 given me facilities for doing so, I elucidate the point by the aid of 

 a diagram, exactly copied, four times the size of his own figure. 

 a, Fig. 1, is the front lens of the object-glass; h, h, rays; these, 

 after being refracted by the convex and flat surfaces, would con- 



