12 



ELEMENTARY CHEMICAL MICROSCOPY 



- Diaphragm 



that the position and diameter of the diaphragm in the eyepiece 

 greatly influence the character and size of the field lens image, 

 and are thus largely responsible for the area of the field of the mi- 

 croscope, and consequently are very closely associated with the 



resolving power of the optical 

 combination employed. The 

 light rays leaving the eye lens 

 are concentrated within a tiny 

 circle, known as the eye-point, 

 eye-circle, Ramsden disk, or 

 Ramsden circle. The desig- 

 nation " eye-point" has been 

 given to this smallest bright 

 spot of light, since it is the 

 proper position for the pupil 

 of the eye when looking into 

 the microscope. If either 

 above or below the eye-point, 

 light rays are lost and the 

 image is less bright and less 

 clear. The diameter of the 

 eye-point is dependent upon 

 the numerical aperture of the 

 objective and the magnifica- 

 tion of the microscope. It will 

 be found upon measuring the 



FIG. 2. Path of Light Rays in a Negative *. r ^.T. i 



E * }ece y diameters of the eye-circles 



produced by different oculars 



with the same objective, that they are inversely proportional 

 to the magnification obtained and that with different objec- 

 tives and one and the same eyepiece, the diameter of the eye- 

 circle varies directly as the numerical aperture of the objectives. 

 The value of the numerical aperture in any consideration of 

 the probable performance of different objectives of the same 

 equivalent focus has already been alluded to. We now see that 

 there is a close relation existing between numerical aperture and 

 the performance of the ocular; for example, of several objectives 



