OF THE MICBOSCOPE. 87 



The room in which the test is made must also be a subject of 

 careful selection. Very many of our best microscopes are used 

 in our large cities; at least they are very generally examined 

 there with a view to purchase. Now, those who are familiar 

 with the subject, know that during the day time the buildings 

 along the principal thoroughfares in our large cities are in such 

 a state of constant vibration, that geod results are rendered im- 

 possible, and therefore that an objective and stand which, 

 under such circumstances, fail to resolve difficult tests, or to 

 define clearly, should not on that account be condemned. 



.The illumination employed must also receive careful atten- 

 tion. An objective which readily resolves the P. angulatum by 

 central illumination, when lamp light or good daylight is used, 

 may fail when poor daylight is employed. Special directions 

 on this point are given under the head of light and illumiii- 

 tion, and therefore we would merely say here that an objec._ . j 

 which has been tested only by the dull blue light of a northern 

 sky, cannot be said to be inferior because it has failed either in 

 resolving or defining power. 



On the other hand, we must not place too high an estimate 

 on an objective which, by the aid of monochromatic light, (the 

 blue-cell, for example,) has resolved certain difficult tests. It 

 is not uncommon to find that lenses of a quarter-inch focus 

 will, with blue light, resolve the AmpJiipleura pettucida, but 

 fail completely with ordinary light. Even eighths and tenths 

 by the same makers, and of a grade quite as good as the fourths 

 just mentioned, fail to resolve the Amphipleura by ordinary 

 illumination, even when well managed. The aid which is 

 derived from blue light in the resolution of difficult diatoms is 

 unquestionable, but it is not quite so clear that this kind of 

 light gives the same assistance in the matter of definition. O'.ir 

 own experience leads us to believe that the real assistance de- 

 rived in the latter case is very slight. Therefore, we do not 

 regard it as a very high recommendation for ordinary work that 

 a lens can resolve the AmpJiipleura by blue light. We have, 

 however, seen a fourth which would resolve the Amphipleura by 

 the light of an ordinary hand lamp, aided by Wenham's reflex 

 illuminator. The objective was made by Tolles, and manipu- 

 lated by him. 



