238 HOW TO WORK 



" The plate-holder is moveable backward and forward on the 

 walnut frame on which the microscope stands, its maximum distance 

 from the stage of the microscope being nearly nine feet. 



" To permit ready focussing at distances greater than the length of 

 the arm, a wooden rod three fourths of an inch in diameter and 

 capable of easy rotation runs the whole length of the right side of the 

 frame. The milled head of the fine adjustment of the microscope is 

 grooved, and a grooved wheel in the end of the rod permits the two 

 to be connected with a band. The operator standing at any part of 

 the frame can therefore manipulate the fine adjustment by simply 

 turning the wooden rod in his fingers. The arrangements of light, 

 position of object, coarse adjustment, &c., are made by the operator, 

 who stands by the microscope, which has a suitable eye-piece 

 adjusted, and observes the object in the usual way ; afterwards re- 

 moving the eye-piece and going to the plate4iolder, the final focus is 

 made by means of the wooden rod, the image being viewed with a 

 focussing glass on a piece of plate-glass held in the same frame which 

 is to receive the sensitive plate. 



" The cell containing the ammonio-sulphate of copper hangs out- 

 side the shutter over the hole by which the light is admitted. It not 

 only excludes the unnecessary illuminating rays, but prevents danger 

 to the objective from the concentrated solar heat, and permits the 

 eye of the operator to view the objects about to be copied without 

 fatigue or injury. Latterly a plate of alum has also been used to 

 exclude solar heat, especially during any .temporary removal of the 

 ammonio-sulphate cell. The chemical processes employed are well 

 known to all photographers. With the above apparatus, it has been 

 found that the best defined pictures are obtained when the distance 

 employed with any objective does not exceed three or four feet. 



" The achromatic concave used as a substitute for the eye-piece, is 

 a combination of somewhat more than half an inch transverse diame- 

 ter, and about 28 angle, constructed like the objective to focus the 

 chemical rays. It increases the magnifying powers of the objective 

 about seven times. It has been found to perform well with both the 

 i-th and T ^th. 



" In photographing the soft tissues or other objects in which illumi- 

 nation with parallel rays produces interference lines, the ground glass 

 is to be placed between the mirror and condenser. Of course there 

 is considerable diminution of light, but this can be overcome for the 

 higher powers by condensing the sun's light on the ground glass by a 

 bull's-eye, or other similar contrivance. If the interference lines as 

 seen by the eye do not disappear with one thickness of ground glass, 

 two or more may be used." 



