24 THE USE OF THE MICROSCOPE 



position and focus of their centers. If this is not the 

 case, the instrument tires the eyes more or less; and 

 double vision (diplopia) may persistently occur. 



These binocular magnifiers are made for an observer 

 with normal sight as regards accommodation for near and 

 distant objects. If this is not the case, the observer should 

 wear glasses, usually distance glasses. This is also specially 

 important in case of astigmatism. Small centered correc- 

 ting glasses may be fitted over the top lenses of the 

 instruments, and are better than spectacles. 



A binocular magnifier with a magnification of about 

 3 or 4 times is useful, or even necessary, for making draw- 

 ings of microscopical preparations; the outhnes being 

 drawn with the camera lucida, and the details put in from 

 the microscope while the drawing paper is under the 

 magnifier. Some such binocular magnifier is almost 

 essential for comfortably dissecting parts of animals or 

 plants in preparing objects for the high- or medium-power 

 microscope. It is practically superior, for this work, to a 

 corrected hand lens. 



Summary.— In this chapter it is explained how lenses 

 are corrected and more or less freed from their chromatic 

 and spherical errors. The advantages to be gained by 

 using such corrected lenses are set forth. A method of 

 compensating the unoccupied eye by a disc of ground glass 

 is given, and this is shown to allow comfortable work 

 with a single corrected lens. The kinds of lenses to be used 

 are described, and the importance of field of view and of 

 aperture is stated. The correct methods of using lenses 

 are given, together with the losses which follow the use of 

 wrong methods. Instruments affording binocular vision 

 with the lowest powers are described. Two suggestions are 

 offered : (1) the use of a lens holder which provides a translu- 

 cent screen for the unoccupied eye, and (2) trying out an 

 erecting low-power binocular with parallel tubes and 

 variable stereoscopic effect, made by the combination of 

 Swan and Porro prisms. 



