8o On Microscopical Measurement. [Sess. 



box ; and if you adhere strictly to the same length of tube, the 

 stage micrometer is not needed again. 



Although this is one way, and perhaps the best, the size of 

 objects may be got in another way, and one giving equally good 

 results. In this method only a stage micrometer is required. 

 First, the image of the object must be projected down, and 

 drawn, or at least the outline, upon paper or cardboard, while the 

 microscope is in a horizontal position. The drawing is effected 

 by means of a camera lucida, a simple form of which may be 

 a thin cover-glass attached to the eyepiece end of the micro- 

 scope, at an angle of 45 \ You can do this by taking, say, a 

 pill-box, which will fit the eyepiece end of the instrument, and 

 bending back the bottom of the box to the above angle. 

 Making a hole in it fully a quarter of an inch in diameter, 

 lay the cover-glass on the inside sloping surface. Then, 

 looking perpendicularly down through the thin glass, the image 

 of the object will be reflected up into the eye, while a virtual 

 image will be projected down on the paper. On taking a 

 pencil, this image may be drawn on the paper. Now remove 

 the object from the stage of the microscope, putting in its 

 place the micrometer, and, as was done with the object, draw 

 the micrometer lines, and you have the size of the object shown 

 by these lines. In doing this, it is not necessary to place the 

 microscope at any particular height above the paper, because 

 the object and the lines are precisely at the same distance, and 

 the value of each is the same. 



Secondly, How to ascertain the magnifying power of your 

 microscope. To do this, only a stage micrometer is required. 

 You put this on the stage, and inclining the microscope as 

 before, using the camera lucida or cover-glass, fitted as above 

 described, project the image of the micrometer lines down on 

 the paper. In doing so, the proceeding is totally different 

 from that described under the first heading. In this case, the 

 eye lens of the microscope must be placed at precisely ten 

 inches from the paper. If you do this, and use, say, a half- 

 inch object-glass, with, of course, any eyepiece, the xoi) tn °^ an 

 inch on the stage micrometer will cover one inch on an 

 ordinary rule. You then know that, with that particular 

 object-glass and eyepiece, the magnification is 100 times. 

 If the xlJT> tu °^ an ^ ncn covere d a space on the paper 



