178 THE MICROSCOPE. 



ground. An ^th, y^th, or a ygth, is then substituted for 

 the ^ inch lens ; and since the angle of aperture of either 

 of these lenses exceeds 140°, the whole field will be illu- 

 minated with a very oblique pencil of light. By careful 

 manipulation of the prism, and the closest attention to the 

 due adjustment of the object-glass itself, the transverse 

 markings on the valve (which are by far the most difficult 

 to resolve) should now come out with great clearness. 



"The reason for using the ^ inch lens in the first 

 instance, is to find, by ' Maltwood,' or otherwise, the 

 particular object desired, and to determine the exact 

 position of lamp and prism ; by commencing to work with 

 the i^th or iVth these would be impracticable. The light 

 of the lamp is now to be shut off, and a side prism (b) 

 inclined parallel to the plane of the stage-plate, and 

 brought as near to the under-side of the object as the 

 form of the fixed stage will admit, placed in position. 

 The light of a second lamp (e) is then caused, as before, 

 to fall very obliquely on the object, and the longitudinal 

 markings will be readily shown. On restoring the light 

 of the front lamp (c), and paying particular attention that 

 neither light shall be in excess, the valve will be brought 

 out in squares, with a sharpness and a delicacy of tracing 

 which I venture to assert will bear comparison with the 

 best results obtained by use of the expensive modern 

 condenser. I would by no means undervalue the advan- 

 tages to be derived from the employment of the condensers 

 now furnished by our eminent makers ; but, seeing that 

 these are manifestly articles of luxury, I incline to the 

 opinion, that to those microscopists who are economically 

 disposed, and already possess one rectangular prism, the 

 trifling addition of a second prism and a second lamp will 

 enable them to do good service to the cause of resolving 

 many difficult test-objects hitherto insurmountable, amongst 

 which I would place in the foremost rank Amphipleura 

 pellucida (N. Acus), the markings on which I do not 

 yet despair of seeing ' crossed ' by some enthusiastic 

 observer." 



We have witnessed with surprise the ease with which 

 difficult markings are brought out by this mode of illumi- 

 nation. 



