338 



F. J. CHESHIRE ON SIMPLE METHODS OF 



mately the upper focal plane of the eyepiece ; and L-L, the plane 

 of the light source. Fig. 5 shows the path of the light rays, 

 by which the retinal image of the axial point of an object on 

 the stage is formed ; and Fig. 6 similarly shows the path of the 

 light rays proceeding from the axial point of the scale in the 

 condenser, and the eye-ring being examined with a hand- 

 magnifier. 



Now, since the condenser has a greater N. A. than the objective, 

 it follows that the cone of light transmitted by the condenser is 

 more than sufficient to fill the objective, i.e., an outer annulus of 

 the condenser opening is non-effective. The efiective opening, so 

 far as the objective is concerned, is limited to that diameter S,* 



which transmits the maximum cone the objective can take up, and 

 the N.A. of this cone is equal to the working N.A. of the con- 

 denser, that is, the diameter S divided by twice the focal length 

 of the condenser. 



Now the length S is projected into the upper principal focal 

 plane of the objective, and also into the plane of the eye-ring, 

 in either of which it may be read off. The first method 

 of reading off this length S, I will distinguish as the back- 

 focus method, and the second as the eye-ring method. In the 

 back-focus method the length S may be read off by fitting the 

 lower end of the draw-tube with a low -power objective, to make, 

 with the eyepiece, an auxiliary microscope. In the eye-ring 



* In Fig. 5, S is shown as a radius in error. 



