Mr. Tolles " Experiments on Angular Aperture." 85 



FICl 



verge in the dry lens at an angle of 170°, as shown by the dotted 

 lines. Now let us introduce water between the lenses. Instantly 

 a change takes place ; 

 no one, I think, will 

 have the hardihood 

 to maintain that the 

 rays will hold their 

 former position : on 

 the contrary, they 

 will converge at an 

 angle of near 80°, and 

 meet at a point in 

 the body of the under 

 lens. They cross and 

 reach the lower hemi- 

 spherical surface ; * 

 here they are re- 

 fracted more out- 

 wards, and form an 

 angle with each other 

 of 108°, only two 

 degrees short of the 

 110° given by the 

 experiment. This 

 angle is obtained by careful projection of the rays, and is the one 

 mistaken by Mr. Tolles for the representative of the extreme rays 

 of the immersion objective. The microscope body is presumed to 

 have been rotated on a sector as usual, but in order to make the 

 thing more plain to those who do not perceive such matters very 

 clearly, we will suppose the Hght itself to be traversed instead 

 (which is very frequently done) ; it must be evident that this will 

 have to be moved a greater distance, on account of the bending 

 outwards of the rays by the lower surface of the hemispherical lens, 

 and so give a false indication of increased aperture. 



The loss of aperture on balsam-mounted objects was demon- 

 strated by me on correct optical laws known ages ago, and I am 

 astonished that in the nineteenth century anyone can dare to dis- 

 pute it as a fact ; but as it seems necessary to give an experiment 

 to convince the sceptical that when the refraction of the front 

 surface of an object-glass is partly neutrahzed by water contact, or 

 destroyed by balsam, and the angle of aperture most woefully 

 reduced on objects immersed therein, I have done this as follows : — 



* A slight displacement of the ray, too small to be shown in the diagram, 

 will be caused by the water-film ; but this does not alter the angular direction 

 in the hemispherical lens, though it may slightly increase the angle on finally 

 leaving it. 



