MICROSCOPIC OBJECTS. 33 



be equal, then the image is ten times larger than the object, and the 

 dimensions of the object are ten times less than indicated by the micro- 

 meter. If the divisions in the micrometer and in the magnified scale 

 be not equal, it becomes a mere rule-of-three sum ; but in general this 

 trouble is taken by the maker of the instrument, who furnishes a table 

 showing the value of each division of the micrometer for every object- 

 glass with which it may be. used. 



CHAPTER III. 



ON THE METHOD OF VIEWING AND ILLUMINATING 

 MICROSCOPIC OBJECTS. 



21. The art of illuminating microscopic objects is not of less 

 importance than that of preparing them for observation. No 

 general rules can be given for adjusting the intensity of the illumina- 

 tion to the nature and character of the object which is to be examined ; 

 and it is only by a little practice that this art can be acquired. In 

 general, however, it will be found that very transparent objects require 

 a less degree of light than those which are less so : and that objects 

 which reflect white light, or which throw it off from a number of lucid 

 points, require a less degree of illumination than those whose surfaces 

 have a feeble reflective force. 



Most opticians have remarked, that microscopic objects are com- 

 monly seen better in candle-light than in daylight, a fact which is par- 

 ticularly apparent when very high magnifying powers are employed ; 

 and we have often found, that very minute objects, which could 

 scarcely be seen at all by daylight, appeared with tolerable distinct- 

 ness in candle-light. An argand lamp, of somewhat peculiar construc- 

 tion, as represented in the accompanying figure 19, is therefore usually 

 employed. It is made so that the body can be raised or depressed 

 at pleasure, and fixed by a screw. Over the chimney-glass a tube of 

 blackened tin is fitted, so as to allow the light only to pass through a 

 single aperture. A large detached lens also, moving on a slide (fig. 20) 

 is sometimes used to throw parallel rays upon the mirror, and this is 

 generally looked upon as the best artificial light. 



