FOCOMETRY AND APERTOMETRY. 



337 



to give a sufficiently orthoscopic image of an object placed in 

 its lower focal plane, which in this form is outside the lens 

 system, and therefore accessible. The following experiment 

 can be made to show this : Take a disc of wire gauze of 

 about 30 to the inch, and place it in the stop-ring of the 

 condenser. Screw an objective, say ^ inch, on to the nose of 

 the microscope, and drop a stop with a hole in it of about 

 Yjj inch in diameter into a low-power eyepiece. Then rack up 

 the condenser until it practically touches the objective. Upon 

 now examining, with a hand-magnifier, the image of the gauze 

 in the eye-ring (that disc of light just above the eyepiece), it 

 will probably be found to have slight barrel-shaped distortion. A, 

 Fig. 4. 



r/c. 4. 



If now the condenser Ls racked carefully down, the lines first 

 become straight (B), and then pass into waist-shaped distortion 

 (C, Fig. 4). 



The rectilinear or orthoscopic image B can be obtained in this 

 way with all dry objectives that I have been able to test. Its 

 attainment with the object gauze at the lower principal focus of 

 the condenser is a necessary preliminary to a trustworthy deter- 

 mination of an N.A. by the condenser-scale method, now to 

 be described. Let us, for the sake ^ of simplicity, represent the 

 optical elements of a compound microscope — the bull's-eye, 

 sub-stage condenser, objective, and eyepiece — conventionally by 

 a single lens each (Figs. 5 and 6). A- A is the lower focal plane 

 of the condenser and the plane of the stop-ring ; B-B, the object 

 plane and the lower focal plane of the objective ; C-C, the 

 upper focal plane of the objective ; D-D, the lower focal plane 

 of the eyepiece ; E-E, the plane of the eye-ring, and approxi- 



