USE OF THE HAND MAGNIFIER 23 



For the magmfications of 3 to 10 considered in this 

 chapter, the lowest powers of the twin-objective (Green- 

 ough) binocular are excellent. This instrument (Fig. 11) 

 consists of two compound microscopes set at a small angle, 

 with a set of erecting (Porro) prisms below each eyepiece. 

 Special low-power instruments of this kind are now made 

 with large eyepieces and, hence, possess the presumably 

 desirable quahty, for low-power work, of a large field of view. 

 These low-power binoculars may well be detached from 

 their stands, and carried in the hand, being used Uke a magni- 

 fying lens. 



A miniature Greenough binocular is now made by 

 Leitz, to be worn like spectacles. These three magnifiers 

 give (1) erect images, (2) binocular vision, and (3) stereo- 

 scopic effect. The value of erect images and binocular 

 vision is not to be doubted. The scientific value of stereo- 

 scopic effect is, however, sometimes doubtful, and it 

 does not seem as if other desirable quahties should be 

 sacrificed to get it. Thus a fourth low-power binocular 

 might perhaps be made by the combination of a Swan 

 half-silvered cube with Porro erecting prisms. Such a 

 binocular would have erect images, but only a decided 

 stereoscopic effect when the eyepiece circles were partly cut 

 off internally by the irises of the eyes, by making the 

 distance between the eyepieces slightly smaller than the 

 interpupillary distance. This binocular would have parallel 

 tubes, and objectives of higher aperture than the Greenough; 

 and would permit of any power objective being used. A 

 trial made by the writer seemed to give satisfactory 

 results. (Perhaps the instrument could be made with only 

 four reflections on each side.) 



In these four binocular magnifiers, the distance between 

 the centers of the top lenses must be adjusted within a 

 millimeter to correspond with the interpupillary distance, 

 for the best vision. The correct distance should give the 

 maximum of light to each eye, and also a complete field for 

 each eye. The maker should have arranged the two 

 fields to correspond exactly, both as to extent, and as to 



