THE MICROSCOPE STAND 



111 



Fig. 104. 



pass thiougli and part to be reflected into the second tube of the 

 microscope &s shown by the dotted lines, thus the full size beam 

 goes to form each image and no lack of resolution occurs ; two 

 perfect pictures are produced with maxi- 

 mum detail, one in each eye. 



It may be expected by those who 

 have not followed the vast improvement 

 that has recently taken place in optical 

 manufacture that the effect of light 

 passing through the prisms woidd injure 

 the quality of the image. This is not 

 the case ; the flat surfaces can be polished 

 without an error of one-millionth of an 

 inch, and no optical designer now hesi- 

 tates to make use of prisms in optical 

 instruments even of the most exacting 

 requirements. 



As the transparency and reflecting power of the surface E A Equal 

 (Fig. 104) can be regulated according to the amoimt of silver that ho^*' 

 is deposited, the relative intensity of each image can be made 

 identical, and the right- and left-hand images are equal in briUiancy. 

 As to the intensity of the mental impression, it has been urged 

 that when an initial body of light is divided into two brilliant 

 parts and one part is sent into each eye of the observer, the effect 

 of brilliancy is the same as if the whole light be directed into one 

 eye only. Certainly there is some reason for this argument, 

 though it may be an over-statement of the case. It is, however, 

 no disadvantage if a slightly stronger light is required with a 

 binocular than a monocular microscope. The monocular ob- 

 server, in order to more readily concentrate his attention on the 

 employed eye, is apt to use an illumination that is far too brilliant, 

 to the detriment of his eyesight. In the use of the binocular, 

 both eyes are equally stimidated, and there is no temptation to 

 use excessive illumination, and theory goes to show that a low 

 illumination is more efficient for displaying fine detail. 



The diagram of the binocular prism (Fig. 104) shows that the Equal 

 distance from the surface E A, where the beam of light is divided p^Jj^Jor 

 into two portions, to the two eyepieces is not of equal length, both 

 the light on the right-hand side has to "*^°'' 

 travel a distance G H farther than the 

 light that passes directly through. It 

 would, therefore, not be possible to focus 

 both beams of light to the same points in 

 the two eyepieces; if this were not com- 

 pensated, one image would be out of 

 focus when the other was sharp. Fig. 105 shows how placing 

 a plate of glass in the path of a beam of light converging to 

 a focus at A has the effect of extending the focus to B, and 



A B 



Fig. 105. 



