18 Wenham, on a Binocular and Single Microscope. 



for on bringing out the markings with the maximum dis- 

 tinctness for the reflected vision, the direct will be found 

 deficient. On altering the mirror, equal distinctness can be 

 obtained in the direct tube, at the expense of the other. If 

 each tube, therefore, has its own independent mirror, this 

 inconvenience will be obviated. 



The adjustment for difference of distance between the 

 eyes is effected as before, the draw tubes being at the 

 minimum limit of proximity when close in, and by drawing 

 these out to a small extent they accommodate for all positions 

 of eyesight. This answers so well in practice as to need no 

 amendment. It will be seen that the reflected rays have 

 further to travel to reach the eye-piece (the radius of each 

 tube being the same) . The distance is just equal to that which 

 they have to traverse ^.cross the interior of the prism ; this 

 causes a slight disagreement of focus between the two, 

 which may be compensated for by drawing out the maia 

 tube about a quarter of an inch more than the other, but it 

 would be preferable to make a small difference in the mag- 

 nifying power of the eye-pieces, which can be simply done by 

 an alteration of distance between their lenses, each eye- 

 piece to be marked for its appropriate tube. By transposing 

 them such an adaptation would often compensate for those 

 whose eyes diflier materially in focus, or one being long and 

 the other short-sighted, which is a common defect. 



The base into which the prism slides rotates to a small 

 extent, for reflecting the image level with the centres of the 

 eye-tubes ; this is the only adjustment, and when set right is 

 held fast by a binding screw in the side of the inner fine 

 motion tube. The prisms having two opposite reflecting 

 surfaces, possess the common property of such, that, how- 

 ever much tilted, the direction of the ultimate emergent ray 

 cannot be altered. Great care and nicety is, therefore, need- 

 ful in working them to the, exact angles for the definite 

 direction in which the ray is to be finally reflected, but this 

 having been properly obtained, gives the double-reflecting 

 prism this advantage, that it cannot be readily put out of 

 adjustment. Fig. 2 is an enlarged outline of the prism ; a 

 ray of light, a, passing through the base, is totally reflected 

 by the surface, b, towards c, at which surface it is again 

 totally reflected in the direction required. Both the inci- 

 dent and emergent surfaces of the prisms must be perpen- 

 dicular to the direction of its corresponding ray, as any 

 refraction is objectionable, and the reflecting sides be ar- 

 ranged considerably within the angle of total reflection 

 (which, for crown glass is about 48°) . The base of the prism 



