xxviii INTRODUCTION. 



getting inside- the rim which carries the lens may lead to the 

 separation of the two lower lenses from each other, rendering 

 the objective useless. 



VII. ON DBAWING WITH THE CAMERA LUCIDA. 



In making a drawing of an object seen with a microscope, 

 it is often found difficult to draw all parts of it on the same 

 scale of magnification ; and as a result, such drawings are 

 liable to give a false idea of the form of an object, and of the 

 relative sizes of its parts. 



This source of error may be obviated by the use of a 

 camera lucida, by means of which a sheet of paper laid on the 

 table at the side of the microscope is visible at the same time 

 as the object under the microscope. The magnified image 

 appears to lie upon the paper, and by tracing out its lines with 

 a pencil, an accurate outline is easily obtained. As, however, 

 the use of the apparatus involves the loss of a large propor- 

 tion of the light from the object, only outlines should be so 

 drawn, and the details should be filled in after removal of the 

 camera from the microscope. 



VIII. ON MEASURING MICROSCOPIC OBJECTS. 



The eye-piece micrometer is a plate of glass, the upper 

 surface of which, is either engraved with a scale, or divided 

 into a number of equal squares by cross-lines ruled upon it. 

 This is placed between the lenses of the eye-piece of the 

 microscope in such a position that the scale can be seen dis- 

 tinctly on looking through the microscope. 



The stage micrometer is a slide on which a scale is en- 

 graved, divided to, say, hundredths of a millimeter. 



To measure a minute object, focus it under the micro- 

 scope with the eye-piece' micrometer in position ; read off the 

 apparent size on the scale of the micrometer ; replace the 

 slide by the stage micrometer and read off upon that the 

 actual value of the noted number of divisions of the eye-piece 

 micrometer. 



