no 



WENHAM, ON THE BINOCULAR MICROSCOPE. 



highest powers. This does not arise from any defect iu the 

 principle or in the construction of the prism, for this, if 

 neatly and properly made, divides the pencil with a knife- 

 like precision and with no appreciable loss, for the total re- 

 flection is perfect and 'effective quite up to the sharp edge. 

 The obscuration of part of the field is caused by the long dis- 

 tance of the l)ack lens from the prism, and it will be found 

 experimentally that when this distance is increased by 

 adapters, or otherwise, a still greater portion of the field is 

 lost. In the low powers, including the -xjth, the posterior or 

 conjugate focus is long, and the back combination of large 

 diameter; consequently, in a relative sense, the prism is com- 

 paratively much closer to the back lens, and any small section, 

 of the field cut off is beyond the limits of the lowest eye- 

 piece. The annexed figure will demonstrate the reason of 

 this dark portion appearing, as it does, more or less with the 

 highest object-glasses, a is the object-glass of a compound 

 microscope taken as a semilens for the simplicity of the illus- 

 tration. The rays from this half, if unimpeded, will fill the 



whole of the field-lens (^) of an 

 ordinary Huygenian eye-piece, 

 but supposing a stop on the prism 

 (which acts precisely as a stop 

 for either half) bisecting the aper- 

 ture be raised from contact with 

 the back of the object-lens to the 

 position c, it then appears that the 

 rays from the diameter cannot 

 reach the side of the field -lens at 

 d; and as we continue to raise 

 the stop, with its edge to the axis, 

 more and more of the field of 

 view and of the object is cut off, 

 and finally, when the prism arrives 

 close to the eye-piece, we should 

 get merely half the field of ^aew 

 and half the object in each eye, 

 and in this position it would be 

 impossible to see the same spot iu 

 the object, Avith both eyes at once, 

 with any of the object-glasses; the 

 object being simply divided into 

 two separate portions. 



It is obviously impracticable 

 to bring the lower face of the 

 prism quite close to the back of the object-glass; but I 



