SOME SPECIAL PKOCESSES 267 



incident light that is, by light impinging in a direction at right 

 angles to the surface of the specimen is required, a simple 

 appliance, consisting of a piece of optically worked glass held 

 in front of the objective at an angle of 45, may be used. In 

 such a case it is better to use objectives corrected to work with 

 covered objects. The image is in this case formed through 

 the reflector, and where an objective is corrected for use 

 without a cover-glass it may possibly result in some deteriora- 

 tion of the image. Since, however, this method is only 

 applicable for use with very low powers it may happen that 

 no serious disturbance results. As to the illuminant, any of the 

 ones already described may be used ; but unquestionably the 

 electric arc lends itself best to this class of work. Unless the 

 illuminant is of a high power, it will result in very lengthened 

 exposures. Incidentally also, the difficulty of focussing 

 when the image is projected on the ground-glass screen 

 of the camera, and of ensuring even and efficient illumina- 

 tion, is very much increased unless the illuminant is one of 

 high power. Colour-screens may be used exactly as in work 

 with transmitted light, except that some observation will be 

 necessary to determine the exact effect of the colour -screen in 

 use. When dealing with reflecting surfaces the effect on the 

 image of a given screen is often very different to what it is by 

 transmitted light, and due regard must be paid to this point 

 when setting up the object. 



The Photography of Tube- and Plate-cultures of Bacteria. 

 Although this process does not directly come within the scope 

 of photo-micrography, yet it so often becomes necessary to 

 obtain representations of such objects that no apology is offered 

 for giving a description of the methods employed. 



The apparatus required consists of an ordinary photo- 

 graphic camera with a suitable lens. The former may be 

 any good camera of the stand or field pattern, and the lens 

 should preferably be of fairly long focus say from six to 

 eight inches and will not require to work at a large aperture. 

 It need not be of an expensive type, so that a modern anastigmat 

 is unnecessary, although of course it may be utilised if availabK 

 All the photographs illustrating this section have been taken 

 with a rapid rectilinear lens of six inches focal length, working 

 at an aperture of F 8, 



