ILLUMINATING APPARATUS 



35 



Fig. 



24.— No. 3294, Focussing Dark- 

 ground Illuminator. 



ness can be selected for examining living specimens, but mounted 

 objects can seldom be examined. 



To overcome this difficulty, R. & J. Beck, Ltd., have designed Focussing 

 the dark-ground iUuminator (Figs. 24 and 25) with a focussing lurj^Stor."^ 

 adjustment. The upper 

 lens (C) remains in im- 

 mersion contact with the 

 slide, but the reflector 

 (D) can be moved up 

 and down, which raises 

 and lowers the illumi- 

 nated point, enabling 

 any slide of from J mm. 

 to IJ mm. thickness to 

 be used. 



A convenient means 

 of setting the focus for 

 a particular slide is 

 arranged for in the 

 mount of the illumi- 

 nator. Fig. 25 shows 

 the focussing illuminator 



mounted for use on the Standard London Microscope. Turning 

 the lever (C), which projects from the lower portion of the mount, 

 moves the focussing lens (D, Fig. 24) ; in doing this it also 

 moves the pin (A, Fig. 25) up and down, and alters its distance 

 from a flange (B, Fig. 25) on the mount. 



If the lever (C) is moved till this pin (A, Fig. 25) is at its farthest 

 distance away from the ring (B, Fig. 25), the slip that is to be used 

 may be placed between the ring (B) and the pin (A) ; and if the 

 lever (C) be again moved till the pin (A) just clamps the slip, 

 the illuminator will be approximately set to the correct focus 



for this thickness of sHp. The 

 final adjustment may then be made 

 when the object is in position. 



The high-power dark-ground 

 illuminator has been found specially 

 valuable for the examination of 

 living bacteria, rhizopods, and other 

 transparent and unstained speci- 

 mens that are difficult to see with 

 direct light. Such objects, however, 

 due to their transparency, reflect 

 only a small portion of the light 

 that falls upon them, and a strong illumination is necessary. To 

 accomplish this the high-power illuminator is made to produce 

 a very minute image of the source of light, so that all the light 

 may be concentrated on a very small area, almost a point. 



Fig. 25.— No. 3294, Focussing 

 Dark-ground Illuminator. 



