414 3Ir. Howard Gruhh [April 2, 



them roughly polished on the sides, to avoid the chance of having to 

 throw away a lens after much trouble and labour has been spent 

 on it. 



There are only three distinct points to be looked to in the testing 

 of optical glass ; (1) General clearness and freedom from air- 

 bubbles, specks, pieces of "dead metal," &c. ; (2) Homogeneity; 

 (3) Annealing. 



The first is the least important, and needs no instructions for 

 detection of defects, any one can see these. The second is much more 

 important, and much more difficult to test. 



The best test for homogeneity is one somewhat equivalent to 

 Foucault's test for figure of concave mirrors. 



The disc of glass should be either ground and polished to form a 

 convex lens, or if that be not convenient, it should be placed in juxta- 

 position with a convex lens of similar or larger size, and whose 

 excellence has been established by previous experience. 



The lens or disc is then placed opposite some small brilliant 

 light — a small gas flame generally suffices — and at such a distance 

 that a conjugate focus is formed at other side and at a convenient 

 distance. When the exact position of this focus is found, the eye 

 is placed as nearly as possible so that the image of flame is formed 

 on the pupil. On looking at it with the eye in this position, the 

 whole lens should appear to be " full of light " ; but at the slightest 

 movement to one side the light will disappear and the lens appear 

 quite dark. If the eye be now passed slowly backwards and forwards 

 between the position showing light and darkness, any irregularity of 

 density will be most easily seen. 



Of course, like everything else, some experience is necessary. 



The rationale of this is very obvious. When the eye is placed 

 exactly at the focus of a perfect lens, the image formed on the pupil 

 is very small, and the slightest movement of the eye will cause the 

 light to appear and disappear. If the eye be not at the focus, the 

 pencil of light will be larger, and consequently it will require a 

 much greater movement of the eye to cause the light to disappear. 

 Now if any portion of the lens be of a difi'erent density to the 

 general mass, that portion will have a longer or a shorter focus ; 

 consequently while the light flashes off the general area of the lens 

 quickly, it still remains on the defective portions. 



By imitating this arrangement and substituting a camera for the 

 eye and forming the focus of a small point of light on the stop of the 

 lens, I have succeeded in ^photographing veins in glass, and sometimes 

 have found this useful as a record. 



The third point — that of proper annealing — is easily tested by 

 the polari scope. 



For small discs the usual plan is to hold them between the eye 

 and a polarizing plane, such as a piece of glass blackened at back or 

 a japanned surface, and look at them through the facets, using as an 

 analyser a Nichols prism. 



