176 HOW TO SEE WITH THE MICROSCOPE. 



over again, feeling sure that many there are who will 

 not object to some further discussion of the subject. 



In the initial attempt to use the prism, the observer 

 should select an object (balsam mounted) with which 

 he is tolerably familiar. The collar adjustment, also, 

 should have been previously ascertained. The next step 

 is to decide on the proper position of the radial bar, i. e., 

 its distance from axis, the extent of this distance will 

 be demonstrated by the illumination becoming too feeble, 

 the images, also, becoming generally unsatisfactory. The , 

 remedy is, in such cases, to cause the radial bar to approach 

 nearer to an axial position, until the field can be suffi- 

 ciently lighted and the object displayed with tolerable 

 vigor. Thus the operator has the means of " gauging " 

 his objective. The mirror being properly posed, it re- 

 mains to obtain the best possible illumination, which is 

 effected by slight changes of the mirror, condenser, and 

 finally the lamp. When things are generally about 

 right, a little movement of the lamp (grasping it firmly 

 by the bowl), sometimes twisting it to the right or left, 

 so as to get the flame exactly edgewise to the mirror 

 (which is best determined in this way), will result in 

 very nice effects, bringing out the striae on such tests as 

 the Moller test-pla,te in a very satisfactory style. 



Resolutions of difficult test objects are accomplished 

 with this illuminator in a very handsome manner. It 

 is an easy instrument to use, and will adapt itself kindly 

 to the objective, of course; the higher the balsam 

 angle of the object glass, the better the definition. It 

 has, in its ease of adaptation a decided advantage over 



