THE PRESIDENT'S ADDRESS. 19 



distances are first measured by an apertometer, and when these 

 have been tabulated, the crystal takes the place of an aper- 

 tometer. The apparatus required, if one does not possess a 

 petrological microscope, is a Nicol polarizer, of a size not less 

 than that sufficient to fill the back lens of your condenser, (if 

 that happens to be an Abbe it will be rather a costly item), an 

 analyzer, a lens inside the tube of the microscope to convert 

 the instrument into a telescope, and a properly cut crystal. 

 Whether this extra apparatus is more costly than an ordinary 

 apertometer I am unable to say, but it is claimed for the new 

 method that it will measure the aplanatic aperture better than 

 the Abbe apertomer. On page 14 a very pretty experiment is 

 described by Mr. Dick, which visibly demonstrates in a most 

 conclusive manner that a greater angle of light is grasped by 

 an immersion than can possibly be by any dry objective. 



Mr. Dick, it will be noticed, uses the sliders with his Abbe 

 apertometer ; these in my hands have always given very rough 

 and, in the case of large angles, erroneous results. 



A preferable method of using the Abbe apertometer is to 

 rotate the hemispherical disc until the light is extinguished, 

 read off the angle of rotation on both sides of zero, take the 

 sine of the mean reading and multiply it by the refractive 

 index of the apertometer ; the result will be the N. A. of the 

 lens. If in this manner you measure wide-angled condensers 

 you will find curious effects. In some the flame image will dis- 

 appear before the periphery of the back lens has been 

 reached ; this shows that the front lens has not sufficient 

 aperture to fill the back lens of the combination. In others 

 the light, after extinction, will become visible again. In some 

 the image of the flame will continue to be in focus during its 

 passage over a large area of the back of the objective, in 

 others it will begin to go out of focus almost as soon as it has 

 left the centre of the back lens. 



Mr. Allan Dick has fallen foul of that dreadful nightmare 

 of diatom structure interpreters, viz., images in the areola?. 

 Images in all kinds of areolations, whether of diatomic or other 

 structures, follow the laws of those arising from minute per- 

 forations. As this is an important and often misunderstood 

 subject, you will pardon me if it is treated at some length. 



Procure a small piece of tinfoil and a fine-pointed needle, 



