ADAPTING A KODAK FOR WORK IN THE FIELD i6i 



distance must be estimated with considerable accuracy to secure 

 good definition. Such rapid exposures are rarely necessary or even 

 desirable in the case of geologic subjects. 



Anyone desiring to use this device will find that auxiliary lenses 

 of 52-inch, 16-inch, and 8-inch foci will be a suitable series for a 3 A 

 kodak having a lens of about 6.75-inch focus. For the use of others 

 with different kodaks, the following formula is given : 



Fo+i VFo'F,, 



where Fo = focus of kodak lens, Fi = focus of auxiliary lens, and 

 Fo+i = focus of the combination. This formula is only approximate 

 but is sufficiently accurate to guide one in the selection of a series of 

 auxiHary lenses. These lenses can be secured from any well- 

 equipped optician who does his own grinding. They should then 

 be calibrated by focusing on a ground glass in the image-plane and 

 taking readings on the scale. Tables can then be constructed, as 

 shown in Figure 2. 



The extra lenses can be carried in any convenient pill-box. The 

 cost for the three lenses and cell is less than five dollars and the out- 

 fit has been found by the writer during two seasons' field work to 

 give highly satisfactory results. 



