MEDIUM-POWER PHOTO-MICROGEAPHY 



119 



pieces of wood are made about one-sixteenth of an inch thinner than the foot of the 

 horseshoe stand of the microscope. They are fitted together and screwed into the 

 large base-board of the apparatus, as shown in the accompanying diagram (Fig. 62). 

 A fourth piece of wood (covered with velvet) is also shown there, which turns on a 

 screw-down pin at Pi, a washer one-sixteenth inch thick being interposed between it 

 and the piece beneath, and passing over the foot of the instrument, is held by the 

 thumb-screw figured at P2. The axis of the microscope should correspond to the 

 axis of the camera. Swinging the arm round, it is locked at P2, and the microscope 

 thereby firmly fixed. 



SECTION II.— TAKING THE PHOTOGRAPH 



(i) Taking the Photograph with ordinary Direct Light. — We now 



proceed to show how to take a photograph with each low power, and to make the 

 explanation more comprehensive and explicit, it is proposed to take a series of well- 

 known "test objects" as types of the different objects which lend themselves to 

 photo-micrography. 



We commence with the proboscis of a blow-fly, or ordinary blue-bottle, one of the 

 most common test objects for an inch with an A or B eye-piece if using achromats, or 

 an inch with a low-power compensating eye-piece if we elect to employ apochromatics. 

 Either of these eye-pieces will furnish us with two reasonably sized photographs of 

 this interesting object when using a 10-inch camera, but if we desire to photograph 

 the finer details of its structure it will be necessary to substitute a half-inch in place 

 of the inch, because its N. A. being greater it will reveal more delicately the minutest 

 details (see article on lenses, p. 77 et seq.). So, too, we may have to employ a sixth to 

 show distinctly some of the finest hairs which exist on the membrane that stretches 

 between the two lobes of the organ, and we shall consider each separately. 



To commence with, it is necessary to centre the limelight, which means that the 

 brightest point of the lime shall be brought into the centre of the field of view in the 

 optical axis of the microscope. This is effected by the milled-headed screws, which 

 raise or lower the light, or move it from side to side in a manner already described. 

 This finished (with the microscope horizontal), the lowest power substage condenser 

 (focus about half-inch, and N. A. "48) is placed in such a position as to give critical 

 light (see p. 103). Shut now the iris diaphragm as closely as it will allow, and lower 

 the condenser again until the minute aperture of the iris is seen with a low-power 

 eye-piece and one-inch objective. Most probably the hole will not be central — if not, it 



