200 American Quarterly Microscopical Journal. 



motion; the objective is focused on these lines; it is not neces- 

 sary to use an eye piece, unless the focal length be very long. 

 Yet for true angle, independent of definite length of tube, it 

 would be sufficient to simply focus upon the lines without an 

 eye-piece; the screw (y) may be used for this purpose, or it 

 may be effected by simply sliding the tube h in a. Suppose, 

 now, the eye lens (/) to be over the middle of the arc; on 

 looking through towards the objective, one will see something 

 like Fig. 3, where the outer circle is the periphery of the 

 front lens, the middle one is the image of the diaphragm at 

 the back of the objective; or, if this diaphragm is sufficiently 

 large, the margin of the posterior system; the inner circle is 

 the image of the end of the tube {b^, and within the area of 

 this will be an inverted picture of external objects, crossed at 

 the center by the lines on the glass ; the objective (<^-), and the 

 eye lens (/), forming a sort of miniature telescope, and having 

 the lines as a common focal point : the smaller circle would 

 disappear if the end of the tube {b) was large enough, and 

 there would be but these two — the periphery of the front lens, 

 and the image of the diaphragm or posterior system — and it is 

 with this last we are to deal. A piece of tissue paper, or 

 cap with ground glass, is now put on at b, and immediately a 

 soft light fills the field, and the lines appear like cobwebs 

 stretching across it. The sector arm carrying the lens (/) is 

 now swung around until the intersection of the lines is tangent 

 to the image of the margin of the posterior system or dia- 

 phragm, as in Figs. 4 and 5, which represent the circles, as they 

 would appear with very small angles ; with wide angles, they 

 are foreshortened, as in Fig. 6, where the larger circle is, as 

 before, the margin of the front lens; the next inner one, is the 

 image of the diaphragm, and the smaller (partly obscure) is 

 the bright field still visible, and whi^h gives the exaggerated 

 angle to measurements made in the old way. The sector arm 

 may be swung many degrees farther on each side before this 

 will disappear. I find that the old triple fronts, as made many 

 years ago, by Powell & Leland and by Spencer, show much 

 less difference between the true and false angle than do the 

 modern systems, with more or less thick single fronts. When 

 the fine lines are thus projected on the face of the front lens, 

 they will, as in Fig. 7, mark the extremities of a diameter of 

 the circle, which, if stopped out, would exclude all light from 



