158 W. M. GKAHAM — TfiE I'HOtOGKAtHY OF blFtERA. 



definite relation to the distance from the lens to the ground-glass. These focal 

 distances are found as follows. 



To find the distance between the lens and the insect. 



Eide : — Divide the focal length of the lens by the amount of the magnifica- 

 tion and add to the result one focal length. 

 Example : — ^Focal length of lens, 4 inches ; magnification, 2 (linear) : 

 (4 -f- 2) + 4 = 2-1-4=6 inches. 

 To find the distance between the lens and the ground-glass. 



Rule : — Multiply the focal length of the lens by the amount of the magnifi- 

 cation and to the product add one focal length. 

 Example : — Focal length of lens, 4 inches ; magnification, 2 (linear) : 

 (4 X 2) + 4 = 8 4- 4 = 12 inches. 

 For a focal length of 4 inches and a magnification of 2, as in the above 

 example, the lens should be 6 inches from the insect, and the ground-glass 

 12 inches from the lens. It will also be evident that to increase the magnifica- 

 tion the lens must be moved nearer the insect, and the back of the camera moved 

 further away from the lens. 



Having placed the front and back of the camera approximately in their 

 relative positions, the picture seen on the ground-glass should consist of an 

 enlarged image of the insect, and below it an equally enlarged image of the 

 millimetre scale. To every negative a free margin of at least half an inch 

 should be allowed. This condition being fulfilled and the images carefully 

 focussed, with the yellow screen in position behind the lens, the amount of the 

 magnification should be ascertained in the following manner. With a pair of 

 dividers measure upon the ground-glass five divisions of the magnified image of 

 the millimetre scale and then find to how many divisions of an unmagnified 

 millimetre scale those five divisions correspond. 



Example : — 5 divisions on the ground-glass scale equal 15 divisions of the 

 unmagnified millimetre scale ; therefore the magnification is 3 times. 



Now stop down the lens xmtil enough depth of focus has been acquired to bring 

 the body, legs and wings into sharp focus, and see that the picture is properly 

 centred upon the ground-glass. 



EXPOSUKK. 



The length of exposure is proportional to the S([uare of the distance of the 

 image (on the ground-glass) from the lens. Now the greater the magnification 

 the greater must be the extension of the camera, and therefore the longer the 

 exposure that will be necessary. The relative exposures requii'ed for varying 

 magnification can be found as follows : — 



1. Find by means of an exposure meter the length of exposure required for a 

 picture at the principal focus of the lens, i.e., for a reproduction of natural size. 



2. Decide upon the magnification desired. 



3. At the chosen magnification, the exposure proportional to that reipiired for 

 an unmagnified picture, as found by the exposure meter, is given by the 

 following formula : — (the magnification + 1)^ -f- 4. 



Example. — It is required to magnify the insect 3 times ; thus 

 (3 + 1)2 + 4 = -\« = 4. 



