212 FOCUSING IN PHOTOGRAPHY [Ch. VII 



excellent way to check up the similarity of level of the ordinary 

 focusing screen and the plate holder. Frequently they do not agree 

 closely enough for the more exacting work, especially in photo- 

 micrography. If the focus is found to be exact proceed to set the 

 focusing glass in a board as follows: 



Have a board of about 15 mm. thickness in a frame like that used 

 for the ordinary focusing screen. Bore a hole in the -center in which 



the focusing glass holder will fit snugly. Now 

 put the frame on the focused camera and 

 slowly twist the focusing glass into the hole 

 until the focus seen through it is perfect. If 

 nothing has changed in the camera then this 

 focus should give perfect results for any 

 future setting of the camera, for the focus 

 will be at the exact level occupied by the 

 sensitive surface of the plate. If it is found 

 Fig. 128. Tripod perfect by trial, it is wise to put some shellac 

 STfSS^gSP « oAer varnish around the mounting to fix 



it firmly in place in the wood so that there 

 will be no change in its position. Of course any change would re- 

 sult in imperfect, out-of- focus negatives. 



This method of focusing has the great advantage of doing away with 

 all obstructing glass. One focuses the position of the real image ex- 

 actly as for a compound microscope when a positive ocular (fig. 22) is 

 used. It is an invaluable way for focusing in photo-micrography. 



§ 335. Objectives and magnification for embryos. — It is a good 

 plan to have one picture of natural size in each case, and then if the 

 embryos or other objects are very small, a picture of 5 or more times 

 natural size. And a picture should go with the embryo or object 

 throughout its entire career so that the exact appearance before sec- 

 tioning or dissection will be available. 



Objectives for making photographs of from xi to X5 range from 

 50 to 100 mm. equivalent focus, and they are placed in the end of 

 the camera as usual (fig. 126). The larger the object the longer should 

 be the focus of the objective; then the exaggerated perspective of 

 short focused lenses will be avoided. 



