ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 569 



object is 30 cm., we shall have for the same object of 10 mm. in dia- 

 meter a drawing having the diameter x , 



a^' 27 , „ 



- = -; .•.^=9mm. 



Thus an object of 10 mm. may, according to the arrangement of 

 the instrument, give at the distance of distinct vision either a drawing 

 of only 9 mm., or one of 13^ mm. These are not exaggerated cases 

 selected for the express purpose of making the variations greater, but 

 are such as are habitually met with, and we may even observe still 

 greater ones. 



The following is one mode of remedying these errors. Eemove the 

 eye-piece and objective (having taken the measure of the amplification), 

 and replace the camera lucida exactly in its original position, that is 

 to say, at the same distance from the stage of the Microscope ; then 

 ascertain experimentally the increase or diminution produced by the 

 image being referred to the paper, and thus by a very simple calcula- 

 tion we can correct the result previously obtained. 



Suppose, for example, that we had found by the old method a 



magnification of 270 times for a given optical system, and that having 



applied the above method we find that 10 mm. referred to the paper 



gives an image of 13^ mm. The magnifying power of the optical 



system having been increased 1 • 85 times, it is therefore in reality 



270 

 1 • 35 times smaller than that which we obtained, i. e. ■- or 200. 



I" 35 



This correction may be applied whatever is the distance at which 

 the drawing is made, and it therefore becomes entirely useless to 

 make it at the distance of distinct vision. The paper should be 

 placed on the table which carries the Microscope, the magnification 

 measured as usual, and then corrected in the mode above mentioned. 



This method has the inconvenience of requiring two very delicate 

 experimental operations, which have to be undertaken at each exami- 

 nation; but by modifying in another way the old method, we may 

 avoid this and obtain directly the magnification produced by any given 

 optical system. 



It is obvious, in the first place, that if the eye-piece and objective 

 are removed, and the paper is placed on the table at a" 6", the draw- 

 ing will be much larger than the object. Since the triangle a" &" is 

 of greater altitude than the triangle A B, its base a" 6" will neces- 

 sarily be greater than the base A B. But if we raise the paper little 

 by little, the drawing will diminish proportionately, and finally a point 

 will be reached where the drawing will be exactly the same size as 

 the object. It is the point where the two triangles O a 6 and A B 

 become equal, where their altitudes being equal their bases are equal 



The position being found in which the drawing and the object are 

 equal in diameter, this equality continues whatever the changes in the 

 tube of the Microscope. In raising or lowering the tube we increase 

 or diminish by equal quantities the altitude of the triangles A B 



