156 SELECTION AND USE 



and polished, the rays will be totally reflected therefrom, and 

 will be sent to the point, b, outside the cylinder. If, however, 

 any insect scales, diatoms, or thin flat objects should be in 

 contact with the upper surface of the slide, the rays will be 

 enabled to pass through, and the objects will be brilliantly 

 lighted up on a dark ground. Objects seen in this way show 

 many points of their structure with remarkable clearness. 

 Speaking of the Amphipleura pettucida, Mr. "Wenham tells us 

 that it assumed a substantial appearance not seen in any other 

 way, and at once displayed its striae with an th that had never 

 resolved them before. 



It is very evident that the results which we have just de- 

 scribed will only take place when a stratum of air lies above 

 the slide, and the objects which we wish to view must be in 

 contact with the slide, and not mounted on the cover, as is 

 usually the case. But if the objects be mounted in balsam, 

 the light will pass through to the upper surface of the cover, 

 whence it will be totally reflected down, and the objects will 

 be invisible, or, if sufficiently substantial, will appear on a 

 black ground with dry objectives. But if we use an immersion 

 objective, the light will no longer be totally reflected at the 

 surface of the cover, but will pass through, and will be taken 

 up by the objective provided the latter has sufficient angular 

 aperture. 



Used under these latter conditions, the reflex illuminator 

 constitutes a means of obtaining oblique illumination of great 

 purity and force, and the instrument is now a general favorite 

 for this purpose, for which it was first used by Mr. Samuel 

 Wells, of Boston. 



To use the reflex illuminator successfully demands great care 

 and some experience, but those who will carefully study its 

 construction and mode of action, will find no difficulty in 

 getting good results. First of all, then, we must remember 

 that the reflex acts as a condenser, and consequently it is 

 necessary to use but a moderate amount of light, which should 

 be as intense as possible. This is best obtained from a small 

 hand lamp. Secondly, we must use parallel rays, or the points 

 to which the light is brought to a focus will not be those which 

 accord with the other features of the instrument. Parallel rays 



