POWELL AXD LE ALAND'S HIGH-POWER BINOCULAR 105 



towards the axis by making the openings of different width and 

 different distance apart. 



With diaphragms of this form (which can easily be made out of 

 cardboard) the larger aperture angles of high-power objectives may 

 be made use of to intensify the stereoscopic effect without employing 

 wide pencils, which are prejudicial both as diminishing the clem-ne 

 of the image and the focal depth. 



Of course with this method of illumination both eye-pieces must 

 be half covered in order that one image may receive light only from 

 one of the two illuminating cones, and the other only 

 from the other. The division of light in both the aper- 

 ture-images will then be as shown in fig. 83 ; and it is 

 evident that in this case the brightness of the image for 

 both eyes together is exactly the same as would be given FIG. 88. 

 by one of the two cones alone without any covering. 



The method of illumination here referred to which was origi- 

 nally recommended by Mr. Stephenson for his binocular microscope 

 has. in fact, proved itself to be by far the best when it is a question 

 of using higher powers than about 300 times. It necessarily requires 

 very well corrected and properly adjusted objectives if the sharpness 

 of the image is not to suffer ; but if these conditions are satisfied it 

 yields most striking stereoscopic effects, even with objectives of 2 mm. 

 and less focal length, provided the preparation under observation 

 presents within a small depth a sufficiently characteristic structure. 



Non-Stereoscopic Binoculars. The great - comfort which is ex- 

 perienced by the microscopist from the conjoint use of both eyes has 

 led to the invention of more than one arrangement by which this 

 comfort can be secured when those high powers are required which 

 cannot be employed with the ordinary stereoscopic binocular. This 

 is accomplished by Messrs. Powell and Lealand by 

 taking advantage of the fact already adverted to, that 

 when a pencil of rays falls obliquely upon the sur- 

 face of a refracting medium a part of it is reflected 

 without entering that medium at all. In the place 

 usually occupied by the Weiiham prism, they in- 

 terpose an inclined plate of glass with parallel sides, 

 through which one portion of the rays proceeding up- 

 wards from the whole aperture of the objective pas,-e> 

 into the principal body with very little change in its 

 course, whilst another portion is reflected from its sur 

 face into a rectangular prism so placed as to direct it 

 obliquely upwards into the secondary body (fig. 84). 

 Although there is a decided difference in brightness be- 

 tween the two images, that formed by the reflected rays 

 being the fainter, yet there is marvellously little loss of FIG. 81. (1865.) 

 definition in either, even when the 50th of an inch objec- 

 tive is used. The disc and prism are fixed in a short tube, which can 

 be readily substituted in any ordinary binocular microscope for the 

 one containing the Wenham prism. Other arrangements were long 

 since devised by Mr. Wenham, 1 and subsequently by Dr. Schroder, 

 1 Transactions of the Microsc. Soc. N.S. vol. xiv. (1866), p. 105. 



