MICROSCOPIC ILLUSTRATIONS. 289 



the angle formed by the margin or edge of their acting 

 aperture with the acting focal point. 



There are a variety of methods of effecting this object 

 thus, if we know the acting focus and the acting aperture, 

 we have only to make a diagram on paper of them (which 

 should be on a magnified scale for greater exactness), and 

 measure the angle with a protractor. In general, it will 

 perhaps be most advisable to calculate the angle of aper- 

 ture for parallel rays, (and for magnifiers this method is 

 the only correct one.) In this case the angle will always 

 be a fixed one, whereas if it is taken for diverging rays, 

 every alteration in the length of the body of the engiscope 

 will cause some trifling alteration in the length of the 

 anterior conjugate focus, so that the angle of aperture will 

 always be less with a short than with a long body. 



A very material point to be predetermined in this mode 

 of measurement is the real focus of the lens, &c. to be 

 measured ; and as few persons know precisely how to settle 

 this in an effective practical way, the following method is 

 given as pretty exact : A triple object-glass must be pro- 

 cured, having a focus of some length, which may there- 

 fore be very exactly measured (reckoning from the middle 

 of its thickness) : it will be most convenient if its focus 

 will come out in a round number, say twenty or thirty 

 inches. The lens, whose focus we want to know, is to be 

 applied to it as an eye-piece, and adjusted for distinct 

 vision on a star, then with a good dynameter measure the 

 power, which we will say turns out to be twenty, with a 

 twenty-inch object glass. 



It follows, therefore, that the focus of the eye-glass must 

 be one inch. N.B. If this method is too troublesome, get 

 an optician to measure some particular lens for you. 



