ARTIFICIAL LIGHT. 193 



suitable object on the stage, with the mirror in a central 

 position, the collar of the objective being set at the ex- 

 tremity of its range. Now, by degrees, swing the radial 

 bar carrying the mirror, meanwhile adjusting the mirror 

 so as to secure all the illumination possible, just as would 

 occur in arranging for greater obliquity of illumination, 

 until the obliquity of the light becomes as great as the 

 objective will bear; i. e., until the greatest degree of 

 obliquity has been obtained that will secure a tolerably 

 well-lighted field. Now move the radial bar a little, 

 and but a little further from axis, meeting this change 

 by the proper manipulation of the mirror, and so as 

 still to secure all possible light. The object now ought 

 not to have more than one-fourth the usual illumination, 

 but should nevertheless be distinctly seen. Next, revolve 

 the collar and notice the effect. If you get less and less 

 light as the collar is turned towards the other extremity 

 of its range, it would show that it was already at the 

 point of its maximum aperture ; on the contrary, should 

 you get more light, it will be apparent that the aper- 

 ture increases as the collar is turned, and thus turning 

 the collar by degrees, move also the radial bar still fur- 

 ther from axis, manipulating the mirror as before, and 

 to the same end, and so proceed as long as the change 

 of the collar gives more light. You have then, by 

 simple inspection of the position of the adjustment, a 

 tolerable idea where the maximum of the objective is to 

 be found. Note this : Now remove the object, place the 

 stand in a horizontal position, and, without changing 

 the adjustment of the objective, proceed to measure its 



13 Microscopy. 



