39 



In will be seen by an inspection of the table that about 35 diameters 

 can be obtained by using the 70mm lens and a camera extension of five 

 and a half feet, or by using a 35mm lens and a 4 projection eyepiece with a 

 camera extension of about 28 inches, or by using the 16mm lens and 

 no eyepiece with a camera extension of 20 inches; each of these methods 

 has of course its advantages, and disadvantages; the first gives a wider 

 field than the last and a deeper focus. Fig. o was made in this manner; 

 with the 16mm lens and no eyepiece only so much of the same object 

 could be taken as lies betw'^een the points a and b in Fig. 3. The advantage 

 this arrangement has to compensate for its smaller field and less deep 

 focus is its greater resolving power; this principle holds whatever the 

 combinations that produce any given power. 



LEVELINtJ. 



The tables and the benches mxist all be exactly leveled; this is easily 

 done by means of a spirit level and the adjustable feet on which they all 

 rest. The cooling cell and condenser must also be level. 



THE ILLUMINATION OF THE OBJECT. 



(a) Centering. 

 It is necessary that all parts of this apparatus be most carefully cen- 

 tered. There are several good ways to do this. One is to place in every 

 piece of tlie optical apparatus a pinhole diaphragm, which may be cut 

 from black cardboard to fit each separate piece, one for the microscope to 

 be substituted for the eyepiece, one for the Abbe and the field diaphragm, 

 unless these parts are already provided with iris diaphragms, in which 

 case they can be shut to a pinhole; one for the biconvex lens and one 

 for the condenser. The instrument is sufficiently centered when a ray of 

 light passes through this series of holes and falls on the center of the 

 ground glass, when the camera is fully extended; these diaphragms should 

 be saved so that proof of the centering can at any time be quickly made. 



(b) The Image of the Light. 



In order to make a good photomicrograph with an objective of 8mm 



focal length or less the image of the light should be thrown into the 



plane of the object. This can, the books say, "with no great difficulty," 



be effected by slipping the light and the condensing lenses back and forth 



