364 



♦ KNOWLEDGE ♦ 



[May 1, 1885. 



number of half or whole wave lengths, and the beautiful 

 phenomena of interference takes place, some of the colours 

 of white light being obliterated while others come to the 

 eye. When the position of the eye changes the colour is 

 seen to change. I have not time to dwell further on this 

 part of my subject, which is discussed in most advanced 

 works on physics, and especially well described in Dr. 

 Eugene Lommel's work on the " The Nature of Light." I 

 remarked that if the two surfaces were perfectly plane 

 there would be one colour seen, or else colours of the first 

 or second order would arrange themselves in broad parallel 

 bands, but this would also take place in plates of slight 

 curvature for the requirement is as I said, a film of air of 

 equal thickness throughout. You can see at once that this 

 condition could be obtained in a perfect convex surface 

 fitting a perfect concave of the same radius. Fortunately 

 we have a check to guard against this error. To produce 

 a perfect plane three surfaces must be worked together 

 unless we have a true plane to commence with, but to make 

 this true plane by this method we must work three toge- 

 ther, and if each one comes up to the demands of this most 

 rigorous test, we may rest assured that we have attained a 

 degree of accuracy almost beyond human conception. Let 



Fig 8(1. 



me illustrate. Suppose we have plates 1, 2, and 3, Fig. 8a. 

 Suppose 1 and 2 to be accurately convex and 3 accurately 

 concave, of the same radius. Now it is evident that 3 will 

 exactly fit 1 and 2, and that 1 and 2 will separately fit No. 

 3, but when 1 and 2 arc placed together they wUl only 

 touch in the centre, and there is no possible way to make 

 three plates coincide when they are alternately tested upon 

 one another than to make perfect planes out of them. As 

 it is difficult to see the colours well on metal surface?, a 

 one-coloured light is used, such as the sodium flame, which 

 gives to the eye, in our test, dark and biight bands instead 

 of coloured ones. When these plates are worked and 

 tested upon one another until they all present the same 

 appearance, one may be reserved for a test-plate for future 

 use. Here is a small test-plate made by the celebrated 

 Steinheil, and here two made by myself, and I may be 

 pardoned in saying that I was much gratified to find the 

 coincideoce so nearly perfect that the limiting error is much 

 less than 00001 of an inch. My assistant, with but a 

 few months' experience, has made quite as aocui-ate plates. 

 It is necessary, of course, to have a glass plate to test the 

 metal plates, as the upper plate must be transparent. So 

 far we have been dealing with perfect surfaces. Let us 

 now see what shall occur in surfaces that are not plane. 

 Suppose we now have our perfect test-plate, and it is laid on 



a plate that has a compound error, say depressed at centre 

 and edge and high between these points. If this error is 

 regular the central bands arrange themselves as in Fig. 9. 

 You may now ask, how are we to know what sort of 



Fig. :-. 



surface we have. A ready solution is at hand. The bands 

 always travel in the direction of the thickest film of air, 

 hence, on lowering the eye, if the convex edge of the bands 

 travel in the direction of the arrow, we are absolutely 

 certain that that part of the surface being tested is convex, 

 while if, as in the central part of the bands, the concave 

 edges advance, we know that part is hollow or too low. 

 Furthermore, any small error will be ligorously detected, 

 with astonishing clearness, and one of the grandest quali- 

 ties of this test is the absence of " personal equation " 

 for, given a perfect test-plate, it won't lie, neither will it 

 exaggerate. I say, won't lie, but I must guard this by 

 saying that the plates must coincide absolutely in tempera- 

 ture, and the touch of the finger, the heat of the hand, or 

 any disturbance whatever will vitiate the results of this 

 lovely process ; but more of that at a future time. If our 

 surface is plane to within a short distance of the edge, and 

 is there overcorrected, or convex, the test shows it as in 

 Fig. 10. If the whole surface is regularly convex then 



Fis. 10. 



concentric rings of a breadth determined by the approach 

 to a perfect plane are seen. If concave, a similar phe- 

 nomena is exhibited except in the case of the convex 

 the broader rings are near the centre, while in the 

 concave they are nearer the edge. In lowering the eye 



