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original surface of the plate. In a direction parallel to the 

 plate the glass is seen to be very strongly veined; and, in 

 any case, unless cheapness is an important factor, it is unde- 

 sirable to put optical work on a glass of this quality. 



3. The defects ol heterogeneity and veins differ only in 

 degree. In the latter case, the change in index is sufficiently 

 sharp to distort the rays passing through the glass so that the 

 distortion is readily visible to the unaided eye. 



Considerable difference of opinion exists as to whether the 

 very broad and subtle forms of homogeneity asserted by 

 some workers do actually exist. It has been held by some 

 that, if a piece of optical glass is free from visible veins and 

 in a properly annealed condition, it is only necessary to work 

 the block accurately flat, or to the proper curvature, to 

 produce perfect results. Others assert that all glass is 

 subject to inherent want of homogeneity, which, in work of 

 the highest precision, may necessitate correction by figuring 

 the otherwise perfect surfaces. 



These two views are not completely contradictory, owing 

 to the fact that want of homogeneity (certainly when it 

 exists in the grosser form of veins) is very frequently accom- 

 panied and made evident by stress, which is produced in the 

 glass owing to difference in coefficiency of expansion between 

 adjacent portions of the material, it is probable that the 

 truth lies somewhere between the two statements, and that, 

 whilst gradual slight changes in index sometimes exist, in 

 many cases the distortion of the wave front produced by 

 some blocks of glass, which has been put down to inherent 

 heterogeneity, is really due to variation of index produced 

 by the stress which has not been removed to a sufficient 

 extent by the annealing operation. 



Three photographs are given of a large pentagonal block 

 which contains rather broad veins, which, however, can only 

 be detected by the unaided eye on very careful examination 

 and with previous knowledge of their exact location. 



The first photograph makes the veins evident by the 

 condition of double-refraction set up in the block owing to 

 the stresses produced in the annealing of the non-homogeneous 

 material. 



The second photograph is taken without the intervention 

 of polarised light, utilising Topler's " Schlieren Methode " for 

 the examination of fine departures from homogeneity in a 

 medium. In this method, light from a small source is 

 brought to focus by a suitable optical system (a concave 



