The Microscope 311 



e. Ease of manipulation, focusing, etc. With the camera 

 extended so as to give an image two and one-half inches in diameter, 

 it will be found that all parts of the microscope (stage, mirror, coarse 

 and fine adjustments, etc.) can be readily reached and manipulated 

 by the hands while the eye is watching the image on the ground 

 glass. For greater enlargements, the focusing can easily be gov- 

 erned by cords and weights, as in the more expensive outfits. The 

 ease with which the prism- tube and eye-piece can be substituted for 

 one another prevents any difficulty in finding and fixing a desired 

 field. 



/. Almost any camera or light-tight box capable of receiving a 

 dry-plate holder may be used, thus doing away with the necessity of 

 a special camera box of extra length. 



g. Cheapness, as compared with other means to the same end. 

 The chief expense is in the prism, which must be both clear and 

 accurately ground. A plane mirror, fixed in the plane CE (Fig. 1), 

 would lessen the cost considerably, but the image given would not 

 be so clear, on account of the unavoidable halation and double 

 reflections from such a mirror. It may be well to state here that, 

 unless the prism is the full size of the vertical tube A, it will be 

 necessary to insert a diaphragm (with either round or square open- 

 ing) in the tube A to shut off the rays of light that would other- 

 wise pass between the sides of the prism and of the tube; and that, 

 in this case, the field given in the image will be to'^the whole field 

 seen through the eye- piece, as the area of the opening in the dia- 

 phragm is to the area of the tube of the microscope. 



h. The appliance may be used as a camera lucida, with or 

 withovit the eye-piece, by simply tipping the microscope to the hori- 

 zontal position. 



The objections that may be urged against the appliance are: 

 That the illumination is by reflected, instead of direct, light; and 

 that the rays lose some of their actinic power in passing through the 

 prism. I think that the combined'disadvantages arising from the 

 above will be found to be theoretical rather than practical, and that 

 they can be fully counteracted by exposing the plate a second or two 

 longer. In fact, I have over-exposed plates in three seconds, using 

 unconcentrated sunlight for illumination and a Zentmayer eight- 

 tenths objective (no eye-piece). 



Figure 2 will show how the camera to be used may be brought 

 up to the same level as the projecting end of the prism-tube. The 

 platform should be of such a height that the image is centered on 



