74 ELEMENTARY PHOTO-MICROGRAPHY. 



attachment to the microscope, the polariser fitting 

 underneath the object slide, and the analyser 

 above the objective. By its use a pencil of light 

 is split up in such a way that the emergent beam 

 is different from the one which enters, or it may 

 be extinguished altogether in certain positions 

 of the prism. This interference of light waves 

 causes certain substances to appear brilliantly 

 coloured, whereby the structure is differentiated in 

 a marked degree. 



It would be difficult, for instance, to get light 

 and shade or variety of form in many substances 

 that are much too transparent for ordinary photo- 

 graphy ; but with polarised light a contrasty 

 negative can be obtained showing the several 

 tissues or structure in a satisfactory manner. 



Slips of selenite, as usually supplied, only give 

 blue and yellow or green and red ; but either a 

 thin sheet of mica or selenite of varying thickness 

 will often intensify the colour effects in a desired 

 manner. It will be found that sober tints yield 

 better negatives than the gaudy ones, therefore 

 the mica or selenite can be selected accordingly. 



A body tube dead black inside is essential, 

 and the most suitable powers are 2in. and lin. 

 objectives. Under all circumstances a great 

 obstruction of light takes place, hence the eyepiece 

 is dispensed with to compensate for the loss 

 through the prism. 



