CINEMATOMICROGRAPH Y 189 



duration of -5$ of a second. In the case of bacteria, blood 

 preparations, etc., magnified 400 diameters and illuminated 

 with a dark ground condenser, an arc lamp or limelight 

 and condenser are essential. Fig. 60 shows a cinemato- 

 micrographic apparatus by Ernemann. The arc lamp 

 should be automatic, otherwise it is difficult to keep the 

 crater at one point for a sufficient time. A condenser 

 and cooling trough are placed between it and the micro- 

 scope. The camera is arranged vertically, and fitted with 

 a handle, by means of which it may be swung rapidly 

 into position after adjustment of the microscope and 

 lamp, and when a suitable field has been found. A side- 

 tube is also provided with an arrangement to fit over the 

 eyes so that the image on the film can be watched during 

 exposure. The mechanism for moving the film can be 

 worked by a handle, but a small electric motor geared to 

 its shaft by a belt is preferable. 



Photomicrography with Polarised Light. Polarising 

 apparatus is indispensable on a microscope intended for 

 petrology, and most makers supply special stands fitted 

 with analyser, polariser, selenites, and completely rotating 

 and graduated stage, etc., built on the general lines of 

 their ordinary stands. These are, of course, most desir- 

 able when any amount of petrological work is done, but 

 by the addition of a polariser to the substage, and an 

 analyser fitting in a nose-piece or above the ocular, almost 

 an}^ microscope can be used for this purpose. Either the 

 analyser or polariser must rotate, and when the axes of the 

 Nicol's prisms they contain are crossed, no light passes 

 unless certain refractive bodies are in the field of view, 

 in which case the play of colours is frequently not only 

 extremely beautiful, but of use in studying the object. 

 This is one of the cases in which colour photography is 

 particularly applicable, and examples are given on Plates 

 1, 10, but even prints in monochrome are useful (Plate 

 25). Obviously the best results cannot be obtained 



