ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



717 



on a separate stand, would take an expci-ienced microscopist a quarter 

 of an hour or more, an iuexi^erienced one an evening.* 



The following are a few hints on dark-ground illumination : — 

 Let us, by way of example, take a definite object, thresh that 

 out thoroughly, then after- 

 wards show what alterations 

 in the method will be re- 

 quired for other objects. The 

 necessary apparatus is an 

 achromatic condenser, and a 

 lamp with a bull's-eye fixed 

 to it. 



It is, in Mr. Nelson's 

 opinion, most important 

 that the condenser should 

 be achromatic. It will be 

 urged by many eminent 

 microscopists that an achro- 

 matic condenser is quite 

 unnecessary. Also there 

 are those who prefer a 

 paraboloid, spot -lens, &c. 

 He does not, however, go 

 into this question for fear of making his jjaper too long ; the 

 scope of it being a method of showing critical images on a dark 

 ground by means of an achromatic condenser ; the test of criticalness 

 being the visibility of the dots in the hexagonal areolation of the 

 larger Triceratia with a 2/3 of 0-21 N.A. (= 32^° air angle). Let 

 us, therefore, take this as our experimental object. 



We must first adjust our lamp and bull's-eye as described on 

 p. 714 and get tlie edge of the lamp expanded to a disk as in fig. 165. 

 Place a small aperture in the condenser, and a Triceratium on the 

 stage with the 2/3 in. objective on the nose-piece. The Microscope 

 having been put in the proper position, the lamp should be placed on 

 the left-hand side of it. The lamj) should now be arranged as to 

 height, so that the rays from the bull's-eye may fall fairly on the 

 plane mirror ; the plane mirror being inclined to reflect the beam on 

 the back of the substagc condenser. 



Now, with any kind of light, focus and centre the Triceratium to 

 the field, fig. IGG. Then rack the condenser until the small aperture 

 in its diapliragm comes in focus ; centre this to the Triceratium, fig. 

 167. Rack the condenser closer up until the bull's-eye is in focus, 

 fig. 168 Here it happens that the bull's-eye is not in centre, and is 

 not uniformly filled with liglit as in fig. 165, but has instead two 

 crescents of light. This is a case which often occurs ; but, of course, 

 it may be more or less filled with light, and m.ay or may not be more 

 nearly centered. 



* Mr. NolHon tliirik.-i it would bo a good jilan if niicjoscopi.stH woukl always 

 nso the term " biill'H-eyo " insbad of "(M-tridcriHor," to doHigiiato that piece of 

 apparatuH ; leaving Uio term corxlonwjr for tiie HubHtagc coinionKcr only. 



