mirror may be conveniently used for this purpose),* and the 

 block to be examined is placed in the path of the rays. The 

 image will be distorted by any heterogeneity of the glass; 

 and when the eye is placed at the focus, or just behind a 

 shutter which is partly cutting off the light at the focus, a 

 variable light-and-shade effect indicates departure from 

 uniformity of index. The very pronounced effect observed 

 in the photograph illustrates the extreme sensitiveness of this 

 method, which will, in fact, detect distortions of the wave 

 front of less than one-quarter of a wave-length, such as 

 would indeed have no influence whatever on the definition 

 of an optical instrument. 



The third photograph shows the irregularity of a plane 

 wave front after passage through the prism block, and is 

 really the most satisfactory method of examining the per- 

 formance of a block so as to sum up the whole optical effect 

 due to heterogeneity, double-refraction and pseudo-hetero- 

 geneity caused by stress. The broadest changes of index are 

 most readily detected by this instrument, which, in the 

 ingenious method worked out by Messrs. Hilger, may be used 

 to indicate exactly how to figure a block of glass to correct 

 this trouble where it cocurs in small amounts. 



A very fine vein causes an altogether inappreciable dis- 

 turbance of the optical image due to diffraction effects, and 

 merely affects the instrument in the same way as small 

 bubbles, that is, by occasioning a minute loss or scattering 

 of light. In fact, the catalogue of optical glass published by 

 Schott & Gen, Jena, stated : " The glass is selected carefully 

 by its naked-eye appearance, and is not rejected on account 

 of the occurrence of fine isolated striae." 



4. Small bubbles which are occasionally present in optical 

 glass, and unavoidably so in certain barium glasses, have no 

 effect on the performance of the instrument but to obstruct a 

 minute portion of the light unless, indeed, they should occur 

 close to the focal plane, as, for example, in the field glass of 

 an eyepiece, in which case they may have an appreciable 

 effect in obstructing a certain portion of the view. 



By a short-sighted policy, much otherwise perfect optical 

 glass has been rejected for minute bubbles which, in the 

 particular position in which the glass is employed, have no 

 effect on the instrument which can possibly be perceived. 



5. " Feather " in a glass, which is made evident by a 

 sheet of minute bubbles occurring locally, near to the edge of 



* The apparatus for this test was shown by Messrs. Ross, at the 

 Optical Society's Exhibition, January, 1917. 



