THE VICTORIAN NATURALIST, 107 



exact focus, the image of the flame, in working with low powers, 

 would only cover a small portion of the field. If it is required 

 that light shall be distributed over the entire field, this can be 

 attained either by racking the substage condenser a little out of 

 focus, or by removing the substage condenser altogether and using 

 only the bull's-eye. Sometimes a piece of ground glass is interposed 

 between the condenser and the object so as to diffuse the light. 

 For some low power work I have found an optical lantern with 

 4-wick lamp answer all purposes, by removing the front pro- 

 jection lens and allowing the 4-inch condenser to project equal 

 rays of light over the surface of the object, in the same manner as 

 it does over a lantern slide ; but it must be remembered that 

 in order to produce a critical image the source of light must be 

 focussed on the object by the condenser. Also, to produce best 

 results it is necessary that the condenser and object glass shall be 

 of equal aperture, or thereabouts. The advantages of this arrange- 

 ment are fully demonstrated in Carpenter on the Microscope, 

 in which it is shown that where the aperture of the condenser is 

 reduced to less than half that of the objective there is a diffraction 

 effect. 



Having, then, centred the condenser and focussed the light, the 

 next step will be to place the light, the microscope, and the camera 

 in their relative positions. The object to be photographed must 

 now be placed on the stage of the microscope and focussed, and 

 the eye end of the microscope inserted in the front of the camera, 

 care being taken to make the connection light-tight. If the image 

 of the lamp-flame has been previously focussed, it should be seen 

 upon the ground glass of the camera, and by its position can be 

 ascertained whether the axis of the camera coincides with that of 

 the microscope. 



The camera seen here has a front board for holding the 

 lamp and microscope, which swings round on a pivot. By 

 this means the preliminary work of ananging the light and 

 focus and centering the object can be done in a convenient manner, 

 and the instrument can then be turned round into position in front 

 of the camera. It will be found that the objective needs to be 

 focussed a little nearer to the object to yield a sharp image on 

 the ground glass of the camera than it did when used for 

 observation with the microscope alone. 



In order to prevent an out-of-focus appearance in the photo- 

 graph, objects should be so prepared that they present one 

 even surface, which shall be in strict coincidence with the 

 optical plane of the lens employed. This, however, can only 

 be done in a very limited numbtr of objects, and in the case of 

 such objects as butterflies' eggs or deep diatoms one must judge 

 for himself whether the best result will be obtained by resolving 

 one plane without paying attention to any other plane, or whether 



