174 TESTING THE MICROSCOPE. 



room. The lamp is then moved to the right and left of the axis, 

 until only half the field of view appears illuminated. The angle 

 at which the extreme positions of the lamp (a and b, Fig. 101) are 

 seen, from the focal point of the objective, is the angle of aperture 

 sought. It is therefore only necessary to place the Microscope in a 

 suitable position, draw lines to the two extreme points right and 

 left occupied by the lamp, and measure the included angle with 

 the protractor. 



The method is still simpler, if the lamp (a few feet distant) 

 remains fixed, and the Microscope is revolved round a vertical axis 

 passing through the focal point, so that the end of the eye-piece 

 describes a horizontal arc. The result will be precisely as in the 

 previous case. The Microscope is turned first to one side and then 

 to the other, in each case until half the field is dark ; in these 

 extreme positions lines parallel to the axis are drawn and pro- 

 duced till they meet at the focal point; the angle which they 

 include is equal to the angle of aperture, and may be measured 

 directly by the protractor. The result is practically the same, if 

 the revolution does not take place exactly on the focal point, but 

 merely on the anterior end of the Microscope. 



If a special mechanical contrivance be desired for such measure- 

 ments, Goring's apparatus is good. This consists essentially of a 

 circular brass plate upon which the Microscope is placed, and 

 which rotates upon a fixed plate that is graduated on the circum- 

 ference; a pointer on the rotating plate enables the observer to 

 read off the angle on the graduated plate. A more detailed 

 description, with illustrations, is given by Mohl in his " Mikro- 

 graphie," p. 193. 



(2.) Wen/iam's Method. 1 The Microscope is placed horizontal 

 and directed towards a distant source of light, e.g., the fiame of a 

 lamp. Instead, however, of observing the field of view in the 

 usual way, the image of the flame is examined with a magnifying- 

 lens above the eye-piece (a'" and I'", Fig. 101). The Microscope 

 is then rotated in a horizontal plane to the right and left, till a 

 certain point of the image for instance, its centre just disappears. 

 The arc, which the Microscope thus describes, may be regarded as 

 corresponding to the angle of aperture of the objective. 2 



1 Mr. Wenham ascribes this method to Amici, vide " Quart. Journ. Micr. Sc." 

 ii. (1854), p. 209. [ED.] 



2 The remark of Harting (" Das Mikroskop," 1st ed. p. 263), that this 



