78 PHOTO-MICROGEAPHY. 



magnesIum-lamp, placing it inside the Lantern instead of the oil- 

 lamp. As the magnesium ribbon is expensive, I use a common 

 fish-tail gas-burner to focus with, brought down into the Camera 

 by an india-rubber tube ; and when the image on the ground 

 glass appears clear and sharp, I take it away and illumine with the 

 lamp to see all is right before exposing. 



If the microscopist works with a one-inch objective, there will 

 be sufficient space between the front lens and the slide to allow a 

 thin piece of wood, covered with black cotton wool, to be held 

 against the lens as a cap ; but if he is working with higher 

 powers, the exposure must be effected by a mechanical con- 

 trivance sold for the purpose. 



Having thus explained the various parts of the apparatus, the 

 next thing will be to put them together and start to work. Great 

 care must be taken to prevent any light being admitted into the 

 Camera except that which passes through the lens; the part 

 where the tube of the microscope fits into the front of the Camera 

 should especially be seen to. The centres of the condenser and 

 magnesium-lamp must also be exactly on a level with the objective. 

 Do not use the focussing-screen belonging to the Camera, as it 

 might not be quite in register, and it is not to be relied on for 

 Photo-Micrography ; but get a piece of very fine ground glass, the 

 size of your plates, and put it into the carrier of the dark slide in 

 the same way as you would the sensitive plate. The mounts 

 selected must be very transparent, — preferably those mounted in 

 glycerine jelly to those in Canada balsam, — and quite free from 

 dust or air-bells. Sections of wood, and the larger species of 

 Algae, are capital things to begin with. Place the slide to be 

 photographed on the stage of the microscope ; take a diaphragm 

 a little larger than the object, so as to allow a margin all round, 

 and fasten it at the back of the stage between the microscope and 

 the condenser (it is shown in position in sketch) ; this will cut off 

 any extraneous light from the lens. Light the gas, which should 

 rest on a movable stand between the magnesium-lamp and the 

 condenser, and begin to focus. A magnifying-glass should be 

 used, and when the picture cannot be improved in sharpness, 

 clamp the Camera to the baseboard. Then remove the gas, and 

 light the magnesium for a moment to see that all is right ; fix a 

 plate in the dark slide, and put the slide into the Camera ; cover 

 the lens, draw the shutter up, and expose. No rules can be laid 

 down for the time to be given, as the conditions vary so con- 

 siderably ; but with plates ten times as sensitive as wet collodion, 

 and a one-inch objective, a minute should suffice. The majority 

 of amateurs expose dry plates too long, whereby they get misty 

 pictures with no contrast. The Pyro-Glycerine developer is the 

 best to use, and the following is Mr. Fry's formula for it; the 



