116 



PRACTICAL TREATISE ON THE MICROSCOPE. 



the drawings have been made of the actual size, but in an 

 inverted position. In fig. 60, c represents a tube of brass, within 

 which a smaller tube, b, carrying the illuminator, d, is moved 

 up and down by turning the milled heads, a a. The tube, c, is 

 screwed into a plate of brass, which turns upon another larger 

 plate ; by this last, the entire condenser is adapted to the under 

 surface of the stage, it being provided with a screw, /, at its 



front part, to regu- 

 ' iiiijlllill late the distance 



that it should be 

 slid in under the 

 stage, so as to bring 

 the illuminator, d, 

 into the axis of the 

 object-glass ; but 

 as the arm sup- 

 porting the com- 

 '^' ■ pound body does 



not move from side to side, the adjustment, to remedy this, is 

 rather more complicated. The brass plate into which the 

 tube, c, screws, is made to turn upon a large pin, fixed to the 

 bottom plate, and by means of a spring and a small raised 

 block of brass, the former is always firmly pressed against the 

 screw, e, as seen in fig. 61 ; when, therefore, this screw is 

 turned, the plate, and with it the tube, c, together with the 

 illuminator, are carried slowly from side to side, and when 

 the exact position is found, the plate may be fixed by the 

 screw, g. 



Mr. WenharrCs Illuminator. — The principle of this instrument 

 consists in placing a dark well or stop behind the object, and 

 causing an intense achromatic light to^pass over and around it at 

 such an angle that no rays can enter the obj ect-glass, consequently 

 the field of view appears quite dark. When a transparent object 

 is placed above this dark well, it wiU be rendered luminous, 

 as it intercepts a portion of the light which passes over the 

 circumference of the dark well, and as we see the object with 

 its own radiant light only, it will appear beautifully illumi- 

 nated in all its natural colours, on a jet black ground. The 



