34 PEACTICAL PHOTO-MICKOGEAPHY 



and with suitable combinations of ocular and camera extension, 

 almost any degree of magnification could be obtained, except 

 of course if large objects are to be photographed. For the 

 latter any good- quality photographic lens preferably an 

 anastigmat might be utilised. 



The initial magnification obtainable with various typical 

 objectives, and also the same lenses in conjunction with certain 

 specified oculars, are indicated in Tables I and II. 



It must be understood that these magnifications only 

 apply when exactly the conditions indicated are fulfilled. 

 They will, however, sufficiently indicate the magnifications 

 that result visually, and are obtained in the position of the eye 

 when using the ocular indicated. The term 'high power,' 

 as applied to an ocular, is a relative one. In visual work, an 

 ocular magnifying eight times is of medium power ; but in 

 photography it is usually regarded as of somewhat high 

 power, since the distance the image is projected has to be 

 taken into consideration. 



Visually, an image is observed immediately above the 

 ocular ; but the projected image is seen at some distance 

 away, with the result that greater amplification is obtained, 

 the degree of amplification depending on the camera-length. 

 The difference between the visual and projected image must, 

 therefore, be borne in mind when speaking of the relative 

 effect of different oculars. 



The magnification resulting from the combination of any 

 of these objectives with any compensating ocular is obtained 

 by multiplying the initial magnification by the number of 

 the ocular. 



With most microscopes an appliance, called a nose-piece, for 

 rapidly and easily changing the objectives, is now supplied. 

 They may be obtained either double, triple, or quadruple, 

 carrying two, three, or four objectives respectively. They are 

 made so that the objectives they carry are screwed in, the 

 nose-piece itself being screwed into position on the microscope 

 where the objective usually goes. This enables the objectives 

 to be successively brought into the optical axis, as required, 

 with rapidity and ease. While they are useful for visual 

 work, they are rarely to be recommended for photographic 

 purposes ; since it is unusual for a nose-piece to be so accurately 



