Pliotograpliing Histological Preparations hy Sunlight. 175 



The introduction of the ammonio-sulphate cell would of itself 

 prevent the passage of most of the heat rays falling upon it, but if 

 this were the only means of excluding them it would not be possible 

 to focus primarily with white light on the cardboard screen in the 

 manner which I have found so convenient. 



I have already stated that the time of exposure required for the 

 production of pictures magnified five hundred diameters or less, 

 was a fraction of a second. With higher powers it increases, vary- 

 ing with the management of the achromatic condenser. For four 

 thousand diameters I have sometimes needed as much as twenty- 

 five seconds. 



So long as the exposure is greater than a second, the requisite 

 time may readily be given with a piece of velvet, or a cardboard 

 screen held in the hand. For shorter exposures some mechanical 

 contrivance is indispensable. That alluded to above seems to 

 answer every purpose, and is arranged as follows : — A wooden 

 screen is fixed between the microscope and the sensitive plate, as 

 close as convenient to the microscope. To prevent side lights reach- 

 ing the plate, the screen is connected with the window-shutter by 

 velvet curtains, which can be turned aside to manipulate the instru- 

 ment, and be let down at the proper time. A circular hole, three 

 inches in diameter, is made in the screen opposite the tube of the 

 microscope for the transmission of the image. In front of this a 

 hght shutter slides loosely up and down, held in place by a cleat of 

 wood on each side, the design being to permit the shutter to fall 

 edge foremost with as little friction as possible. The shutter may 

 be made of thin metal, of wood, or even of cardboard. I am using 

 one of pine wood the xVth of an inch thick ; I have used one of card- 

 board with equal success. In the shutter is an opening, three inches 

 wide by ten long, covered with a cardboard slide, by means of 

 which any width of slit, from a fraction of an inch to ten inches, 

 can be given. The part of the shutter below the slit closes the 

 aperture through which the image passes when the shutter is fixed 

 in place before the exposure is made. On drawing a wooden trigger 

 the shutter is started on its fall, which is arrested by a piece of 

 string of suitable length. The exposure has now been made, but 

 the aperture through which the image passes is again closed, this 

 time by the part of the shutter above the sht. The shutter is so 

 light that the jar caused by the sudden arrest of its motion by the 

 string is too trifling to do any damage to the microscopic apparatus, 

 and as it occurs after the exposm-e is over it cannot affect the image. 

 I find that if when the shutter is started the lower edge of the sht 

 is an inch above the aperture through which the image passes, a 

 convenient velocity is attained for a magnifying power of two to 

 five hundred diameters, arranged as I have described. For still 

 shorter exposures, necessitated by lower powers or other circum- 



