42 THE USE OF THE MICROSCOPE 



Use. — In adjusting a twin-objective binocular fitted with 

 a focusing eye-tube or a focusing objective, the instrument 

 is first focused as a whole for the center of the focal range 

 and the center of the field of view of the fixed tube. With- 

 out moving the rackwork, it is now focused for the center 

 of the field of the second tube, by the focusing eyepiece 

 or objective. (It seems doubtful, however, if this adjust- 

 ment is worth having.) If the two fields do not coincide 

 as to extent, and the sight is not normal as to focus or 

 accommodation, distance spectacles should be tried. In 

 some cases, the fields may be made to coincide (if both 

 objectives are correctly centered) by shifting back one 

 of the prism sets which had been moved in its fastenings 

 in the process of cleaning. Often the lack of coincidence 

 of fields, which is a common fault, is due to the twin 

 objectives in use having come from some other instrument 

 of the same make. 



One defect in some makes of twin-objective binocular 

 is the difficulty of getting at the prisms in their casing, and 

 the objective lenses in their often long tubes, to clean them. 

 A year of use in a laboratory without cleaning may accumu- 

 late much dust and many smears on prisms and objectives, 

 and will deteriorate the image correspondingly. It may 

 also deteriorate the user's sight; for he may tend, uncon- 

 sciously, as the days go on, to employ brighter light in 

 attempts to overcome the fog. If the whole instrument 

 is not covered when not in use, eyepiece caps may be 

 obtained to fit over the eyepieces. 



The centering screws on the twin objectives of the 

 Greenough binocular should not be altered except by the 

 optician. The twin objectives are separately adjusted for 

 each instrument, each objective to its own set of prisms, 

 so that the optical axes meet at the center of the object- 

 field; and, therefore, twin objectives from different instru- 

 ments of the same pattern should not be interchanged. 

 Pairs of matched eyepieces (though not single eyepieces) 

 from instruments of the same pattern by the same maker 



