16 THE USE OF THE MICROSCOPE 



'V\\Q l)cgliiner with the microscope, as every one knows, has troubles 

 because the unoccupied eye persists in seeing. If an opaque shade is 

 ])laced in front of it, or if it is closed, matters are not better. The well- 

 known rule is to keep the unemployed eye open, and gradually to learn 

 to neglect everything it sees. More or less temporary diplopia often 

 results. Also, in the course of years, the unemployed eye commonly sees 

 less and less, and may in time become partially blind. The remedy is to 

 change the eye at the tube; but this change is rarely made, because of 

 initial difficulties. 



If a translucent, but not transparent, screen is placed over the unoccu- 

 pied eye, and the requisite time allowed to get used to it, the following 

 advantages may result: (1) the intensities of the light reaching the two 

 eyes may be roughly balanced by putting a sheet of white paper on the 

 table under the unemployed eye; (2) there is nothing to lead the unem- 

 ployed eye astray, and prevent it from converging with the other, or 

 to keep their axes from being parallel; (3) the accommodation of the two 

 eyes can change together, since the translucent screen prevents the 

 unemployed eye from fixing on near objects; (-i) if it is desired to change 

 the observing eye, the screen may be arranged so that there is a constant 

 reminder as to wdiich eye is to be used. After observing with the right 

 eye for years, it is possible to change to the left eye in a month or two, 

 so that this eye gives images good enough for routine work. 



Such a screen may consist of a circle of glass finely 

 ground on one side. In the absence of ground glass, a 

 smooth piece of waxed paper may be used instead. 



Lens Holder. — The single corrected lens (cemented 

 doublet or triplet), though inferior to a binocular of equal 

 magnification and correction, has a place in scientific work 

 because of its portabihty and durabiUty. There is, in the 

 writer's opinion, no discomfort on using a well-adjusted 

 binocular magnifier for long periods. There is, however, 

 some discomfort on using a hand lens for more than a few 

 minutes at a time. A common method of holding the lens 

 away from the eye is perhaps due to the discomfort from 

 the unused eye being lessened when object and lens are 

 held so far away that the unused eye can focus in their 

 vicinity. It appears that a lens holder is required 

 which not only compensates the unoccupied eye, but 

 also induces the user to hold the lens at its correct distance 

 from the eye. (This distance is 12 millimeters without 

 spectacles; but for spectacle users the lens should be 



