108 



THE MICEOSCOPE 



Fig. 100. 



employed when the illumination was specially arranged so that 

 the whole of the object glass was equally illuminated. If, for 

 instance, a fine oblique feather of light was employed, it would 

 only enter one side of the object glass and consequently only one 

 eye would receive the light, the other eye seeing no image. In 

 like manner, if more light happened to be entering one half 

 of the object glass than the other, the illumination of the two 



eyes was different, often to the extent of 

 making binocular vision inoperative. 



It will be seen that none of these dis- 

 advantages exists in the new instrument here 

 described. 



The second type of binocular was in one 

 respect on the right principle. In all models 

 of this type the beam of light is not bisected 

 into two halves, but the entire beam is filtered 

 into two portions, so that some light from 

 every part of the object glass goes to each eye. 

 The Powell and Lealand (Fig. 100) shows the 

 earliest form, the whole light from the object 

 glass (0) impinges on a glass plate (1) and the 

 major part passes through this thick glass plate, 

 emerging in a direction parallel to and almost continuous with its 

 original direction ; but a percentage is reflected at the first surface 

 and proceeds to the prism (2), which reflects it up a second tube, 

 placed at an angle with the optic axis of the direct beam. 



This type of instrument gives equal resolution to that of a 

 monocular microscope because the size of the beam which forms 

 each image is not reduced. 



With the Powell and Lealand form, however, the tubes of 

 the microscope must be long and the instrument bulky, and it 

 suffers from the very grave defect that 

 the light that is reflected is so feeble as 

 to be insufficient for satisfactory vision. 

 The light in one eye is only of about 

 one-sixth the intensity of that of the 

 other. 



The Abbe binocular eyepiece, which 

 acted optically on the same principle in 

 not bisecting the beam into two halves, 

 but in filtering the light by a reflected 

 and a refracted beam, improved the light distribution, but only 

 made the relative illumination in the two eyes about 1 to 2|, 

 and, while not curing this defect, introduced a farther dis- 

 advantage. The general plan of this eyepiece is shown in Fig. 101, 

 and it will be seen that the light which is split up into two by 

 reflection and transmission at the surface is resolved into two 

 beams, one (A D) which is transmitted, the other (A B C) which 



/ 



Fig. 101. 



