OBJECTIVES AND OCULARS 13 



of approximately the same equivalent focus, but possessing 

 different numerical apertures, that one having the highest aper- 

 ture will permit the employment of an ocular of much higher 

 power and thus yield a considerably greater magnification 

 without loss of detail. 



If an attempt is made to increase the ocular magnification 

 beyond a certain limit the eye-point becomes so small that the 

 image resulting is blurred and indistinct. This fact must be 

 borne in mind in microchemical examinations where high mag- 

 nifications must often be brought about by using high power 

 oculars with low power objectives of long working distance. 



In order that images of satisfactory distinctness and sharpness 

 of detail may be obtained, the optical combination for work 

 must be such as to yield an eye-point not less than one milli- 

 meter in diameter nor greater than the diameter of the pupil 

 of the eye of the observer. 1 The diameter of the eye-point and 

 the position of the plane in which it lies can easily be ascer- 

 tained by holding a piece of thin ground glass or waxed paper 

 over the ocular, shading it with a screen or with the hand and 

 raising or lowering it until the bright circle seen upon the glass 

 or paper attains its minimum diameter. 



Oculars to be used on the chemical microscope should have 

 the plane of the eye-circle at such a distance above the eye-lens 

 as to permit the adjustment of drawing or other prisms to the 

 position of maximum brightness and diameter of field. 



Compensating or Compensation oculars are eyepieces specially 

 designed for use with apochromatic objectives. They are so 

 called because of the fact that they aid in the correcting of chro- 

 matic aberration. 



Oculars are said to be par-focal when they are so constructed 

 as to permit their interchange on the microscope without dis- 

 turbing the focus of the instrument. 2 



Compensating oculars are usually par-focal. 



1 Wright, F. E., The Methods of Petrographic Microscopic Research, Bui. 158, 

 Carnegie Inst. Washington, 1911, p. 38. 



2 For a consideration of the conditions to be fulfilled in their construction, see 

 Gage, The Microscope, p. 47. Tenth ed. 



