323 



♦ KNOWLEDGE ♦ 



[April 17, 1885. 



eminent workers, the Clarks of Cambridge, use a modifi- 

 cation of the Ross method which in their hands is pro- 

 ductive of the very highest results. The device is very 

 simple, consisting of a, telescope (a Fig. 1) in which the 

 aberrations have been well corrected so that the focal plane 

 of the objective is as sharp as possible. This telescope is first 

 directed to a distant object, preferably a celestial one, and 

 focussed for parallel rays. The surface b to be tested is 

 now placed so that the reflected image of the same object, 

 whatever it may be, can be observed by the same 

 telescopa It is evident that, if the surface be 

 a true plane, its action upon the beam of light 

 that comes from the object will be simply to change 

 its direction, but not disturb or change it any other 

 way, hence the reflected image of the object should be seen 

 by the telescope a without in any wav changing the 

 original focus If, however, the supposfd p'ane surf^C:) 



second telescope, a diaphragm is introduced in which a 

 number of small holes are drilled, as in Fig. 2, x, or a slit 

 is cut similar to the slit used in a spectroscope as shown at 

 1/ same figure. The telescope a is now focussed very 

 accurately on a celestial or other very distant object, and 

 the focus marked. The object-glass of the telescope b is 

 now placed against and "square "with the object-glass of 

 telescope a, and on looking through telescope a an image 

 of the diaphragm with its holes or the slit is seen. This 

 diaphragm must now be moved until a sharp image is seen 

 in telescope a. The two telescopes are now mounted as in 

 Fig. 2, and the plate to be tested placed in front of the 

 two telescopes as at c. It is evident, as in the former case, 

 that if the surface is a true plane, the reflected image of the 

 holes or slit thrown upon it by the telescope l will be seen 

 sharply defined in the telescope a. If any error of convexity 

 exists in the plate, the focal jjlane is disturbed and the eye- 



Fig. 1. 



proves to be convex, the image will not be sharply defined 

 in the telescope until the eye-piece is moved au-ay from the 

 object-glass, while if the converse is the case and the 

 supposed plane is concave, the eye-piece must now be 

 moved iou-ard the objective in order to obtain a sharp 

 image, and the amount of convexity or concavity may be 

 known by the change in the focal plane. If the surface 

 has periodic or irregular errors, no sharp image can be 

 obtained, no matter how much the eye-piece may be 

 moved in or out. This test may be made still 

 more delicate by using the observing telescope a 

 at as low an angle as possible, thereby bringing 

 out with still greater effect, any error that may exist in 

 the surface under examination, and is the plan generally 

 used by Avian Clark i- Sons. Another and verv' excellent 

 method is that illustrated in Fig. 2 in which a second 

 telescope b is introduced. In place of the eye-piece of this 



piece must be moved cnif. If the plate is concave it must 

 be moved in to obtain a sharp image. Irregular errors in 

 the plate or surface will produce a blurred or indistinct 

 image, and as in the first instance no amount of focussing 

 wU) help matters. These methods are both good, but are 

 not satisfactory in the highest degree, and two or three 

 important factors bar the way to the very best results. 

 One is that the aberrations of the telescopes must be 

 perfectly corrected, a very difiicult matter of itself, and 

 requiring the highest skill of the optician ; another, the 

 fact that the human eye will accommodate itself to small 

 distances when setting the focus of the observing telescope. 

 I have frequently made experiments to find out how much 

 this accommodation was in my own case, and found it to 

 amount to as much as one-fortieth of an inch. This is no 

 doubt partly the fault of the telescopes themselves, but 

 unless the eye is rigorously educated in this work, it is apt 



