40 THE USE OF THE MICROSCOPE 



Eyepieces and Objectives. — The different pairs of 

 eyepieces of the twin-objective binocular should be parfocal; 

 that is, have their front focal planes at the same position 

 in the microscope tube. For this, appropriate collars are 

 to be fitted, as is done by several makers. If the eyepieces 

 are not parfocal (in this sense), the focus of the microscope 

 will have to be altered each time the eyepieces are changed. 

 The two fields of view will then cease to coincide perfectly. 

 Orthoscopic and similar eyepieces with high eyepoints 

 are doubtless best for eyepiece magnifications above 10 

 times, if such enlargements are needed. 



The twin objectives of this binocular, which are usually 

 cemented doublets or triplets, should also be parfocal, 

 that is, should have their positions calculated so that there 

 will be no need of any marked change of focus when they are 

 changed. Some optical firms now make a revolving 

 changer for the three lowest-power twin objectives (27, 126). 

 The maximum aperture of the ordinary series of twin 

 objectives is about 0.1 and, hence, the maximum useful 

 magnification is near 100 times. (The aperture of any 

 of these objectives can be found by dividing the radius 

 of the object-glass by the distance of its edge from the 

 center of the focused object.) Some optical companies 

 now make objectives for this binocular giving magni- 

 fications of 200 or 300 times, which should entail higher 

 apertures than 0.1. Water-immersion twin objectives are 

 useful for the examination of marine or fresh-water organ- 

 isms, since with dry objectives a layer of water introduces 

 spherical errors, increasing with its thickness. Some 

 optical firms now make a twin-objective binocular with 

 larger eyepieces, thus giving a larger field; which adds to 

 the sense of reaHty in the magnified image, and may be 

 of practical use in dissection (27, 90, 126). 



Illumination. — The twin-objective binocular is best fitted 

 for opaque illumination. This may be obtained from a 

 low-voltage lamp above the stage, with a condensing lens, 

 or pair of lenses, and a screen of dayhght glass. The 

 background may be black or opal glass, or yellow-green 

 glass above a white card. 



