196 PKACTICAL PHOTO-MICKOGRAPHY 



to have a scale placed at the side of the camera showing the 

 extension from the front of the ocular. If an exact scale in 

 inches or centimetres is not adopted, a series of marks 

 showing each ten inches of extension will be found very useful. 

 It is then only necessary to observe the length of camera that 

 is being employed, taking into consideration the ocular and 

 the objective, when by a very simple arithmetical calculation 

 the magnification that is being obtained on the ground-glass 

 screen can be approximately estimated. For most objects 

 such an approximation is sufficient, and it is only in cases where 

 an exact estimate of the magnification employed has to be 

 obtained that other methods need be resorted to. 



If the estimation of magnification is not sufficiently accurate 

 when arrived at by simply considering the focal length of the 

 lens, the power of the ocular, and the camera extension, then 

 recourse must be had to the projection of the image of a micro- 

 meter ruling. This is simply a 3" x 1" glass slip, on which is 

 mounted a cover-glass ruled by a diamond at fixed distances 

 apart. The rulings are generally one-tenth and one-hundredth 

 of a millimetre. It will be best perhaps to carry out the first 

 estimate of magnification by means of a medium-power objec- 

 tive say, one inch. Turn the microscope in the horizontal 

 posit ion, having got the ruling on such a micrometer in focus, and 

 project the image on to the ground-glass screen of the camera. 

 The screen should preferably be at a considerable distance, if 

 the camera will admit of it say, five feet from the front lens 

 of the objective. If the camera extension will not admit of an 

 image being projected to this distance, then it will be sufficient, 

 if the source of light is fairly powerful, to throw the image 

 on to a white screen or card at the required distance. We 

 thus have a projected image at sixty inches' distance, and on 

 reference to this image if we find, say, that the rulings, 

 each of one-tenth of a millimetre apart, when projected on 

 the ground-glass screen, have a distance between them of 

 6 mm., we shall know that the initial power of the objec- 

 tive is ten, and that the lens therefore is correctly a one-inch 

 objective. Similarly, if we take any other lens and divide the 

 magnifying power obtained by six, the result again will give 

 the initial power of the lens at a distance of ten inches. With 

 higher powers it will be necessary to use the one-hundredth 



