OF METALS: F. C. THOMPSON 165 



is better since, in general, a larger field would be obtained. In cases 

 where vibration may make its influence felt, long camera lengths 

 rarely yield satisfactory results, since the extent of the oscillations 

 increases with the length. It would -appear, therefore, that in this 

 case on the few occasions when it is necessary to push the magnifica- 

 tion of a given image as far as possible, it is advisable to use a high 

 eye-piece and short camera extension rather than the reverse. 



In connection with the use of the projection eye-pieces it was 

 discovered that the setting, i.e., the relative positions of eye and 

 field lens, which requires adjustment for different lengths of camera, is 

 dependent not only on this and on the ocular used, but also upon 

 the objective. The effect of the latter is shown to be considerable, 

 and the statement made that " the possibility of movement of a 

 projection eye-piece is practically superfluous," must be admitted to 

 carry weight, although it is at variance with present notions. Four 

 diagrams are given in which the camera length is related to the best 

 setting of the eye-piece for objectives of different focal lengths. 



In conclusion, another direction in which modifications of existing 

 methods might be of value may be pointed out. Cases may welt 

 arise in which extremely oblique rays would reveal a structure in 

 what, when viewed directly, appears to be a structureless constituent. 

 The insertion of suitable diaphragms behind the back lens to effect 

 this would facilitate at times the resolution of a " line " into its 

 component dots. The general rendering of an object would be 

 deleteriously effected by such a procedure, but in certain cases distinct 

 advantages might be gained. 



