264 Transactions of tJie [Sess. 



the same way to spread it over the glass. The film in both cases 

 must be very thin, and just sufficient left on the glass to see 

 that it is there and no more. 



With microscopic objects that polarise well, polarised light is 

 of very great assistance in Photo-micrography. It brings the 

 various markings into greater contrast — -differentiating them in such 

 a vi^ay that they are much more easily photographed, and give 

 better results. The exposure in such a case is, however, much 

 more prolonged. AVhat would require, with transmitted light, say 

 fifteen seconds, with polarised light requires five minutes. 



The demonstration I propose to give you to-night is to photo- 

 micrograph the palate of the Whelk, and to do so by polarised light. 

 After placing the slide on the stage of the microscope, and fixing it 

 in the position I wish it to be, I place the lamp in position, — and this 

 must be carefully adjusted as to the height of the flame, as it also 

 must be placed in the optical axis of the object-glass, or as nearly so 

 as may be. I then place the bull's-eye condenser at the distance of 

 its own focus from the lamp, and between it and the stage. I place 

 the polarising prism under the stage, and on the top of the prism 

 I place a selenite, giving a blue tint, as I wish to produce a strong 

 colour in the object. The analysing prism is, of course, placed 

 over the object-glass. The glass I shall use is a Zeiss's " aa," — 

 what is termed on the glass itself a -fth. It gives one of the 

 flattest fields I have ever seen in any glass, and the most beautiful 

 definition to the very margin of the field. I place the microscope- 

 tube in the hood in the front of the camera, and tie it on. I 

 should perhaps have mentioned before that the tube should either 

 be lined with black velvet or receive a coating of dead black by 

 mixing lamp-black in lacquer and painting the inside of the tube 

 with one coat and setting fire to the spirit. One coat is enough 

 if the mixture is thick enough. This leaves a fine dead black 

 surface, and not a glossy one as when more than one coat is 

 given. The only thing now to do is to see that the illumination 

 is exactly in the centre of the optic axis. To do this, remove 

 the focusing screen and look through the camera and microscope 

 tube. If the image of the flame is seen in the centre of the 

 tube, all is well ; but if not, the lamp must be altered in such 

 a way as to accomplish this end. No pains must be spared in 

 doing this, as upon it depends entirely whether your negative 

 will be a success or a failure. It is very easy to do with low 

 powers, but when using high powers and an achromatic condenser, 

 it is sometimes a matter of ten minutes' work to get the light right 

 in the optic axis. When that is accomplished, and the apparatus 

 firmly secured to the board, and the light fairly in the centre of 

 the screen, you may at once proceed to expose the plate. As I 

 am using now polarised light, I shall give this object — although 



