82 Modern Microscopy 



scope. For those, therefore, who desire to be able to employ 

 both eyes, they make a binocular eyepiece, but only for the 

 Continental length of tube, and this should be particularly 

 understood. The one with which we are acquainted is that 

 designed by Abbe and manufactured by Zeiss, as shown in 

 Fig. 23. In some hands it gives very beautiful results, 

 while other workers have failed to derive advantage from 

 it. It is designed to give stereoscopic effects and to work 

 with both high and low powers. If it were mounted in 

 some lighter manner it would perhaps become more 

 generally used; its weight is very much against it when 

 working with high powers ; still, for the advantages it 

 affords it is an adjunct which is by no means to be despised. 

 BLANK EYEPIECE. It will be found convenient, especi- 

 ally in examining the back of the objective to observe 

 diffraction phenomena, cones of illumination, etc., that a 

 blank or * dummy ' eyepiece be employed ; that is, an 

 ordinary eyepiece mount having no lenses in it. The 

 aperture in the cap must, however, be a very small one. 



STANDARD GAUGES FOR EYEPIECES. 



The following sizes were adopted by the Council of the 

 Eoyal Microscopical Society on December 20, 1899, as the 

 standard inside diameters of draw-tubes for microscopes, 

 the tightness of the fit of the eyepiece being left to the 

 discretion of the manufacturers : 



No. 1, '9173 inch = 23'300 millimetres. 

 No. 2, 1*04 inches = 26'416 

 No. 3, 1-27 inches = 32'258 

 No. 4, 1-41 inches = 35'814 



No. 1 is what is known as the Continental size, which is 

 made almost universally by Continental manufacturers, and 

 has been supplied for many years. It has also been largely 

 used by English manufacturers. 



No. 2 is the mean of the sizes used by the English trade 

 for students, and small microscopes. 



No. 3 is the mean of the sizes used for medium-sized 

 binoculars and for microscopes of a similar class. 



