xxvm INTRODUCTION. 



be cleaned with chamois-leather or silk. If the objective is 

 smeared with glycerin, wash it carefully, then dry with a 

 soft handkerchief. Canada balsam, which sometimes gets on 

 the objective, may be removed with a drop of benzol on a 

 handkerchief. It is, however, safer to leave this to an 

 optician or to the demonstrator, as a very small quantity of 

 benzol 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 DRAWING WITH THE CAMERA LUCID A. 



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 proportion 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 

 distinctly on looking through the microscope. 



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

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



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



