162 PEACTICAL PHOTO-MICKOGKAPHY 



diaphragm-aperture in correct focus ; in fact, if there is any 

 falling off or haziness to one side or the other, it clearly in- 

 dicates that the illuminant is not in its correct position. This 

 preliminary operation will usually be carried out with the 

 microscope on a table, and suitably inclined for visual work. 

 The centration of the light is therefore carried out by moving 

 the mirror until the desired effect is obtained. This adjust- 

 ment having been made, the condenser may now be racked 

 up until the image of the radiant comes into view, and this 

 image should be as carefully focussed as possible if critical 

 illumination is desired. The diaphragm may now be opened, 

 and the degree to which this should be done or not is a matter 

 of considerable moment. The merest novice will soon per- 

 ceive that, with a diaphragm that is closed to a considerable 

 extent relatively to the aperture of the objective, a very great 

 falling off in the quality of the image results. For ordinary 

 purposes, and when observing pathological, botanical, or any 

 similar preparations, the best result is usually to be obtained 

 when the diaphragm is open sufficiently to fill approximately 

 two-thirds of the objective-aperture. The proportion of the 

 objective -aperture that is being utilised may be determined 

 by removing the ocular from the body-tube of the microscope 

 and looking down the tube. If the whole aperture is being 

 employed, the back of the objective will appear to be quite 

 filled with light, whereas when the diaphragm is closed the circle 

 of illumination gradually decreases. The most satisfactory 

 result is usually obtained when the diameter of the circle of 

 illumination is equal to two-thirds of the diameter of the whole 

 visible aperture of the back lens. 



There are very few objectives which will, under ordinary 

 circumstances, stand a larger cone of illumination than this ; 

 but for bacteriological or similar preparations it will be found 

 that a larger cone may safely be used. In photographing it is, 

 from a pictorial as well as from other points of view, necessary 

 to have an evenly illuminated field. This may often be obtained 

 by racking up or down the condenser, and so departing from 

 the conditions that give critical illumination. This, therefore, 

 is a method of dealing with the matter that should never be 

 adopted if any other means of attaining the same result are 

 available. The object of the auxiliary collecting-lens is to get 



