87 



direction will find it somewhat difficult to make a 

 proper selection of apparatus. The most inexpensive 

 is to use the ordinary microscope in a horizontal posi- 

 tion and attach to an ordinary camera, in which care 

 will have to be observed to see that the ground glass 

 is at exactly right angles to the optical axis. As a 

 rule the ordinary view camera is not of sufficient 

 accuracy to use without specially adapting it to this 

 purpose, and the main difficulty lies in the lack of 

 coincidence of the ground glass with the film side of 

 the plate. Plates in themselves are irregular and the 

 shoulders on which they rest should at any rate coin- 

 cide. The best plan to determine this is by placing 

 a straight edge on the frame of the ground glass, 

 interpose a wooden wedge between it and the ground 

 surface of the glass and mark the point of contact by 

 a pencil mark. Follow the same procedure with the 

 plate holder containing the plate. The variation can 

 in this manner be seen with a nicety and proper cor- 

 rection easily made. The writer recommends that 

 this test be made with each plate, so that this is 

 always in exact coincidence. 



Where means will permit, the best plan is no doubt 

 to obtain an apparatus which is complete in itself and 

 is used for this purpose only. The Atwood, as an inex- 

 pensive, and the Rafter as complete in every direc- 

 tion, may be recommended. 



Whether an eye-piece should be used or not is a 

 matter of controversy. Both methods are followed 

 with good success. By the use of the eye-piece the 



