INTRODUCTORY CHAPTER. 13 



the object-glass in which its magnified image is clearly discerned. In order, how~ 

 ever, to effect this adjustment with the greatest nicety, the table is capable of a 

 second motion upon the sliding collar R, by means of the micrometer screw Z, 

 which elevates and depresses the table through minuter portions of space than 

 the pinion V. The motion of the sliding tube G, by the action of the pinion K, 

 is employed for the same purpose, and also for increasing and diminishing the 

 magnifying power with the same eye-piece. 



The instrument is furnished with an assortment of object-lenses and eye-glasses, 

 which can be used either singly or in combination. An object can be seen with per- 

 fect distinctness when it is magnified nearly five hundred times, and superfi- 

 cially 250,000, but when the magnifying powers employed range from one to 

 four thousand, the outlines of the images are no longer well preserved, and they 

 become somewhat obscured. 



In order to enable the observer to measure the size of minute objects, the in- 

 strument is provided with a scale (Q) ruled upon glass. This scale is about the 

 twenty-fifth part of an inch in length, and is divided accurately by the point of 

 a diamond into one hundred equal parts. Each division, therefore, is about 

 ^5 th of T ^-Q th of an - inch, or one-twenty-five hundredth of an inch in length. 

 These minute divisions are clearly discerned, and have each a visible length even 

 under the low powers of the microscope ; and by laying a small object, as a 

 hair for instance, across this scale, its dimensions can be accurately determined ; 

 thus, if the breadth of the hair occupies twenty-five divisions of the scale, its 

 breadth is yf f^th, or one-one hundredth of an inch, and so for any other thick- 

 ness. If, however, the object to be magnified cannot be laid directly upon 

 the scale, its dimensions may be taken by means of the camera lucida, an opti- 

 cal instrument which can be attached at pleasure to the microscope. By its 

 aid the observer beholds at the same time, and in the same direction, the magnified 

 object and the image of a sheet of paper placed upon the table before him. The 

 magnified object lies apparently upon the paper and can be drawn upon the sur- 

 face of the latter under all its enlarged dimensions. The object may then be re- 

 moved, and the scale placed in its stead beneath the microscope, and an image of 

 its magnified divisions also drawn. This drawing being now compared with that 

 made of the magnified object, the dimensions of the latter are readily ascer- 

 tained. Thus, if the drawing of a magnified feather of a butterfly's wing is one 

 inch in breadth, and the length of the drawing of ten divisions of the scale, 

 equally magnified, is also one inch, then the natural breadth of the feather is ten 

 times g^Voth, or one-two hundredth and fiftieth part of an inch. 



SOLAR MICROSCOPE. This instrument is better adapted to popular illustra- 

 tions, than to accurate and delicate investigations ; being far inferior to the 

 compound microscope in the perfection of its construction. 



The essential parts of a solar microscope are a mirror, a condensing lens, and 

 an object-glass. 



The lenses are placed within a brass tube, the larger and open end of which 



