40 HOW TO USE THE MICROSCOPE 



ground illumination came into vogue, what is called 

 a "spot lens " was used for this purpose. It was 

 a lens with a central black circle, fitted in a sliding 

 mount in a collar, which could be slipped into the 

 under-stage and focussed. Spot lenses are not used 

 so frequently as tliey were, because they are prac- 

 tically included in a condenser ; but they are still 

 available, and, having a long focus, are often exceed- 

 ingly useful for illuminating zoophytes in troughs 

 which are beyond the focus of an ordinary con- 

 denser. 



On occasion, a spot lens can be extemporized 

 by sticking a circle of black paper on the flat surface 

 of a bull's-eye. This lens is placed plane surface to 

 the mirror, under the stage, at a suitable distance 

 beneath the object. I have also read somewhere 

 about an ingenious individual who made a rough 

 spot lens out of a buU's-eye taken from an electric 

 pocket lamp. The lens, with black paper disc 

 affixed on its plane surface, was fixed into a circular 

 aperture in a small slab of cork, the highest point 

 of the convex surface being barely flush with the 

 surface of the slab. This device was used on the 

 stage of the microscope, the object being laid above 

 the extemporized lens. 



The Polariscope is an accessory I should not care 

 to dispense with. It consists of two Nicol prisms of 

 Iceland spar suitably mounted ; one prism, known as 

 the " polarizer," is mounted so that it can be slipped 

 into the under-stage collar and rotated by means of 

 a milled head, and the other is mounted so that it 

 can be either screwed on to the nose of the body 

 tube above the objective, or fixed above the eye- 



