148 PEACTICAL PHOTO-MICKOGBAPHY 



only a small point of light is allowed to pass. Should there be 

 any want of centration in the position of the light, this will now 

 become evident, and a much smaller error is at once seen if the 

 iris is closed than when it is wide open. Having effected the 

 centration the iris must then again be opened, but only suffi- 

 ciently to well illuminate the object. The bull's-eye condenser, 

 or a similar auxiliary condensing-lens, may then be interposed 

 between the source of light and the object. It is important to 

 see that this lens is placed so that it is exactly at right angles 

 both in a vertical and horizontal direction with the optical axis. 

 It will be found that it is in its best position when either an 

 approximately parallel or a convergent beam is projected on to 

 the stage ol the microscope, or on the iris -diaphragm underneath 

 the stage. Some movement to and fro of the bull's-eye con- 

 denser will no doubt be necessary to obtain this result. The 

 condition, therefore, is that the illuminant is approximately at 

 the principal focus of the bull's-eye condenser, which should 

 present its plane side towards the light, and the object is 

 evenly illuminated with a parallel beam. The iris- diaphragm 

 on the microscope should then be contracted until no more light 

 than is necessary reaches the object, and so that the size of the 

 illuminated area is no larger than is required. It should be 

 noted that to obtain a parallel beam the bull's-eye must be near 

 to the source of light, and must be moved away from the light 

 to obtain a convergent one. It may be useful to alter the 

 position of the light or of the bull's-eye in relation to the micro- 

 scope and watch the unevenness of illumination that results 

 from any want of centration. It is on this point as much as 

 on any other the correct centration of the light and auxiliary 

 condensing-lens that good results depend in any low-power 

 work. 



It is assumed that the worker is conversant with the ap- 

 pearances of an ordinary projected photographic image, such as 

 may be seen in any ordinary camera and is projected with an 

 ordinary photographic lens. If he is not, however, then the 

 preliminary work had better consist of the observation of such 

 an image without a microscope at all, but using an ordinary 

 photographic lens on the front of his photo-micrographic 

 camera. He may then with any relatively large microscopic 

 object perform the same operation of illuminating the object and 



