20 



THE MICROSCOPE AND ITS ACCESSORIES. 



called a slide. ' Microscopic slides are of different sizes, and are 

 usually oblong in shape. Those in most common use are three 

 inches long arid an inch wide. The object is covered by a very 

 much smaller and thinner glass plate the cover=slip. The whole 



preparation is then placed upon the 

 stage in such a way that the cover- 

 slip is upward and immediately be- 

 neath the end of the tube. The 

 mirror of the microscope is now so 

 adjusted as to concentrate the rays of 

 light on the preparation, illuminating 

 it as much as is necessary. By means 

 of the rack and pinion, or coarse 

 adjustment, the whole tube is now 

 slowly lowered toward the cover-slip 

 until the bare outlines of the object 

 are dimly seen in the white field. 

 From this point on, the micrometer 

 screw, or fine adjustment, is used in 

 bringing the front lens down to its 

 proper focal distance from the prep- 

 aration. The object is now seen to 

 be clear and well defined. By turn- 

 ing the screw to the right or the left, 

 different parts of the specimen are 

 brought more clearly into view, this 

 result being due to the fact that not 

 all points in the preparation are in the 

 same plane. 



In studying objects it is always 

 well to draw them, using a sharpened 

 pencil and smooth paper. The be- 

 ginner soon finds that with constant 

 practice he can sketch the different 

 parts .of the field in nearly their proper 

 relationship. This by no means easy 

 work is facilitated by the use of a 

 drawing apparatus called the camera 

 lucida. The best of these is that 

 devised by Abbe. It is fastened to 

 the upper end of the tube, above the 

 ocular. The apparatus is so made 

 that both the preparation and the 

 drawing surface are seen by the same 



eye. The microscopic field is seen directly, while the drawing sur- 

 face is made visible by means of a mirror. When the apparatus 

 is in place and the drawing commenced, it appears to the one 

 sketching as if his pencil were moving over the preparation itself. 



Fig. 2. .Diagram showing the 

 principle of a compound microscope 

 with the course of the rays from the 

 object (a b] through the objective 

 to the real image (b l a 1 ), thence 

 through the ocular and into the eye 

 to the retinal image (a* i> 2 ), and 

 the projection of the retinal image 

 into the field of vision as the virtual 

 image (b 3 a 3 ). (Fig 21, Gage, The 

 Microscope, eighth edition. ) 



