THE PROJECTION MICROSCOPE 191 



real black and white, but simply different thicknesses in a 

 substance as transparent as glass. When we look through a 

 compound microscope at such a diatom as Arachnoidisctis, for 

 instance, we are looking apparently at an object a few inches 

 across, with a superabundance of light. Now the lantern 

 would project a screen image of even a foot across, as sharply 

 as could be wished ; but a very few feet away from the screen 

 all the detail, though far larger than on the compound, would 

 be invisible ; and when to meet this we have amplified the 

 image to, say, a couple of yards across, the lights and darks 

 are naturally diluted into shades of grey. Again, to get a 

 perfect or ' critical ' image, the photographer stops off a great 

 deal of light, which he can very well afford to do, because he 

 can make up for deficient light by a longer exposure. But 

 the demonstrator cannot do this. Whatever cannot be seen 

 on his screen at the first moment, is seen no better by being 

 left on for an hour. He must have light, and yet his image 

 of any details must be large enough to be seen at a distance. 



On the other hand there is one compensating considera- 

 tion, in that a ' critical ' image is seldom really necessary to 

 him, and can rarely be appreciated at his distances even 

 when produced, except of coarse objects. A certain breadth 

 or coarseness of line is a positive advantage in an image to be 

 viewed many feet away. Fortunately, also, the more serious 

 the work in hand, the more favourable are the conditions. A 

 class of thirty or forty people, really studying a biological 

 subject, would be in a smaller room, and at a smaller screen 

 distance. All could probably get well within 12 feet of the 

 screen, where both more detail can be seen, and the image 

 will be brighter and sharper. Detail can thus be well shown 

 in a class-room, which it would be foolish to attempt with the 

 same illumination before a thousand people. It is the human 

 eye which fails, long before the microscope ; so that an object 

 can be shown quite successfully when the audience have opera- 

 glasses to soe the details with, which the eye alone cannot 



