1898.] MICROSCOPICAL JOURNAL. 135 



used on this so that the ruled lines are sharp, one can 

 readily detect when the image is sharp. 



The extension of camera determines the linear magni- 

 fication of an object, and as half the value of a photo- 

 micrograph for educational jnirposes is dependent upon 

 the degree of magnification being known, it is just as well 

 either to work always with a given extension for each 

 power or to calculate out each time the amplification. 

 " The linear amplification of a projected image is the 

 distance between the image and the posterior focus of 

 the lens system, divided by the focal lengths of the 

 system. The posterior focus of the lens system corres- 

 ponds in the microscope exactly to the upper side of the 

 ocular. It follows from the preceding data that the 

 amount of amplification pf an image for any distance 

 between ocular and screen is found by dividing this 

 distance, expressed in millimetres, by tlie focal length 

 of the objective used, and multiplying ■ the quotient 

 obtained by the number of the ocular " (Van Heurck). 



The initial power of a lens is found by dividing 10 (the 

 nearest average distance of distinct vision in inches) by 

 the focus of the objective, thus 10 divided by J=80, the 

 initial power of i inch. If this be multiplied by the 

 power of the eye-piece, it gives the magnifying power of 

 the combination; thus with an eye-piece magnifying 3 

 times we have 80X3=240 diameters. To apply this to 

 a camera a proportional sum is used : — As 10 : the camera 

 length ; : microscope amplification: camera amplification. 

 Example : Using a i inch objective, eye-piece magnifying 

 three times, and camera extension of 24 inches, required 

 the magnification — 



As 10 : 24 : : 240 : x=576 diameters. 



If the objective alone is used, then the length of the tube 

 must be added. A ith inch on a 10 inch tube and 

 camera extension of 24 inches will give us ; — 



