180 



POWELL AND LEALAND S MICROSCOPE. 



FIG. 7. Small Condensing Lens for viewing opaque objects by direct light, 



/^^~~ : ^. ^e stem of which goes into the 



| r~] /fc==gfa=r3 hole (I). The larger lens should 



M x^rix ^ rst b e ma de to throw as intense 



\ff a light as possible on the objects, 



Fig.7. and the small one should inter- 



cept the rays at about two inches from the object. When the large lens 

 only is used, which is the case when viewing opaque objects by daylight, 

 it should be placed at about three inches from the object, with its plane 

 side towards it : this produces light intense enough for many objects, 

 without the smaller one ; but when that is used, the 

 plane side of the large lens should then be towards the 

 lamp. 



FIG. 8. Diaphramfor cutting off extraneous light when 

 viewing minute transparent objects. It is adapted under 

 the stage, in the same manner as the achromatic condenser. 

 FIG. 9. Diaphram of French construction. Which is used in front 

 of the lamp, to cut off extraneous light, 

 and intercept the rays before they reach 

 the mirror. It goes into the stem of 

 the large condensing lens, which is 

 easily drawn out, and the diaphram 

 substituted. It should be placed at about 

 eight inches from the mirror, and if an 

 image of the aperture is obtained in the 

 body, but a small portion of the field of 

 view will be illuminated. The size will 

 depend upon which aperture is used, and 

 the relative distances of the mirror from 

 the object, and the diaphram from the 

 mirror ; it is useful, when any very deli- 

 cate structure is to be observed, as the 

 darkness around the object renders the markings more distinct. 



Adjusting Object-glasses. These are made to adjust, in order that they 

 may be as perfect for viewing objects covered with glass, as those that are 

 not. As it is of material consequence that the object-glass should ad- 

 just to suit the various thicknesses of glass that are used, and as it is 

 impossible to measure accurately the thickness, the best method to adjust 

 the object-glass is this : Place it at the point for viewing objects un- 

 covered, which will be known by observing that the circular line under 

 the word uncovered corresponds with the fixed line ; or, the more ready 



