MKTHQD OF USING THE MIQllOSCOPE. 327 



2J. On the management of the light depends in a great measure 

 the distinctness of the vision : and as, in order to have this in the 

 greatest perfection, we must adapt the quantity of hght to the nature 

 of the object, and the focus of the niagnitier, it is therefore necessary 

 to view it in various degrees of Hght. In some objects it is dithcult to 

 distinguish between a prominence and a depression, a shadow or a 

 dark marking; or between a reflection of light, and whiteness, which 

 is particularly obser\'able in the eyes of Libcllnla and other insects • 

 all of them appearing very different in one position from what they do 

 in another. The brightness of an object likewise depends on the 

 quantity of the light, the distinctness of vision, and on regulating the 

 quantity to the object; for some will be in a manner lost in a quan- 

 tity' of light scarcely sufficient to render another visible. 



The light of a lamp or candle is generally better for viewing mi- 

 croscopic objects than daylight, it being easier to modify the former 

 than the latter, and to throw it upon the objects with different degrees 

 of density. The best lamp that can be used for this purpose is the 

 one invented by Count Rumford, which moves on a rod, so that it 

 may be easily raised or depressed. The light of a candle or lamp 

 is increased, and more directly thrown upon the reflecting mirror or 

 object, by means of a convex lens mounted on a semicircle and stand, 

 so that its position may be easily varied. If the light thus collected 

 from a lamp be too powerful, it may be lessened liy placing a piece of 

 thin writing-paper, or a piece of fine grayed glass, between the object 

 and the reflecting mirror. Thus a proper degree of light may be ob- 

 tained, and diffused equally all over the surface of an object, a cir- 

 cumstance which ought to be particularly attended to; for if the light 

 l>e thrown irregularly upon it, no distinct view can be obtained. 



The examination of objects so as to discover truth, requires a great 

 deal of attention, care, and patience; with some skill and dexterity, to 

 be acquired chiefly by practice, in the preparing, managing, and apply- 

 ing them to the microscope. 



Whatever object offers itself as the subject of our examination, the 

 size, contexture, and nature of it are first to be considered, in order to 

 apply it to such glasses, and in such a manner, as may show it best. 

 The first step should always be to view the whole together with such 

 a magnifier as can take it in all at once ; and after this the several 

 parts of it may the more fitly be examined, whether remaining on the 

 object, or separated from it. The smaller the parts are which are to 

 be examined, the more powerful should be the magnifiers employed. 

 The transparency or opacity of the object must also be considered, and 

 the glasses employed accordingly suited to it; for a transparent object 

 -will bear a much greater magnifier than one which is opaque, since 

 the nearness that a glass must be placed at, unavoidably darkens an 



