208 PHOTOGRAPHY WITH A VERTICAL CAMERA [Ch. VII 



(i) While the camera delineates rapidly, the image is liable to 

 distortion. I believe opticians are agreed that, in order to obtain 

 correct photographic images, the objective must be properly made, 

 and the plane of the object must be parallel to the plane of the ground- 

 glass. Furthermore, as most of the objects in natural history have 

 not plane surfaces, but are situated in several planes at different levels, 

 the whole object may be made distinct by using a long focus objective 

 and a small diaphragm. 



§ 328a. Papers on this subject were given by the writer at the meeting of the 

 American Association for the Advancement of Science in 1879, and at the meeting 

 of the Society of Naturalists of the eastern United States in 1883; and in Science 

 Vol. Ill, pp. 443, 444. 



§ 329. Scale of photographs. — It is desirable to make all photo- 

 graphs at some definite scale. To do this without much waste of 

 time the camera should be calibrated for each objective that is to be 

 used. This is easily accomplished by using a metric scale like that 

 shown in fig. 104. By lengthening and shortening the bellows of 

 the camera so that the image distance is greater and less one can 

 get the exact position for a group of magnifications and reductions. 

 If the length of the bellows is noted for each size, and the distance, 

 of the objective from the object when the focus is good is also noted 

 one can arrange the camera very quickly for any special size which 

 may be desired. The sizes found very useful by the author are: 

 3; i; 3; 2; 1; 2; 2.5; 4; 5. For magnifications above 5 it is better 

 to make a negative natural size (xi) and then make an enlargement of 

 this, as explained in § 359. 



The vertical camera shown in fig. 126 has the supporting rod gradu- 

 ated in centimeters and half centimeters. After the extension of 

 the camera for any size has been once determined, it is easily made 

 the same at some future time. 



§ 330. Magnification rod for the camera. — Objects vary so much 

 in thickness that the focusing range of the camera should be consid- 

 erable. With the ordinary camera there is usually no provision for 

 moving the camera as a whole for focusing. With the vertical camera 

 shown in fig. 126, where both ends of the camera must be clamped, it 

 is difficult to focus over a large range and keep the length of camera 



