158 W. M. GRAHAM—THE PHOTOGRAPHY OF DIPTERA. 
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. 
Rule :—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+ 2) +4=2 + 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. 
Evample :—¥ocal length of lens, 4 inches ; magnification, 2 (linear) : 
(4 x 2)+4=8 + 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 until 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. 
EXPOSURE. 
The length of exposure is proportional to the square 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 required 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, z.c., for a reproduction of natural size. 
2. Decide upon the magnification desired. 
3. At the chosen magnification, the exposure proportional to that required for 
an unmagnified picture, as found by the exposure meter, is given by the 
following formula :—(the magnification + 1)? + 4. 
Exvample.—It is required to magnify the insect 3 times ; thus 
(3 + 1f~p S44, 
