172 SELECTION AND USE 



be the best results on difficult diatoms, lie knows when his 

 lenses are doing good work in his hands, and, so far as his in- 

 strument is concerned, he feels confident that he can apply it to 

 any class of objects and get views that are trustworthy. 



To get the best results from modern objectives of wide angle 

 of aperture, there are two things that must be carefully at- 

 tended to the illumination of the object, and the adjustment 

 of the correction for the thickness of cover-glass. Of delicacy 

 in focussing, which, by the way, is a most important point, it 

 is unnecessary to speak. 



As regards illumination, the three great points that must be 

 secured are purity, intensity, and suitability. By purity we 

 mean that the light must be wholly of a certain degree of 

 obliquity, for these wide-angle lenses gather in so many rays, 

 that a diffused light, which does not affect low-angled objec- 

 tives, greatly injures the working qualities of those of high 

 angle. Thus, for example, if we are working by central light, 

 it is necessary to shut out all the direct rays from the lamp, 

 which would enter at every sort of angle, and produce con- 

 fusion. A narrow pencil, produced by sending a beam of 

 parallel rays of intense light through a small hole placed some 

 distance below the stage, will give central illumination, which 

 will give very different results from that obtained by a dull, 

 diffused light, such as may be obtained from the flat side of the 

 wick of a lamp, falling on a large mirror and reflected upward. 



When oblique illumination is used, great care should be 

 taken to prevent rays of different degrees of obliquity from 

 falling on the object. A large, concave mirror fills a large 

 portion of the arc through which it swings, and its upper and 

 lower edges reflect rays which fall upon the object with very 

 different degrees of obliquity. Where very 'oblique rays are 

 used, but a small proportion of them enter the glass slide, 

 unless they are " guided " through by some such contrivance 

 as the Eeflex Illuminator, Tolles' Traverse Lens, etc. Kays of 

 less obliquity enter much more freely, and although less in 

 quantity than the others in the first place, they drown them 

 out. In the exclusion of these rays consists in a great degree 

 the value of many of the "illuminators" in common use, and 

 the same effect may be secured to a considerable extent by 



