302 Mr. L. Wright on Optical Combinations 



screen (PI. VI. fig. 1). The series of flat steps or tints are 

 both more conspicuous and more readily understood; and if the 

 two wedges are properly matched, when crossed the diagonal 

 row of squares will be black when the Nicols are crossed. 

 When the thick edge of one is superposed over the thin edge 

 of the other, with the mica axes parallel, we have an even 

 tint; and when the thick, edge is superposed over the other 

 thick edge, with the mica axes crossed, the retardations or 

 colours produced by the first wedge are all destroyed by the 

 second, and the field is all black. I wish to remark here, 

 as I have done elsewhere, that the idea of wedges and other 

 designs, built up in this way of thin flat films, is not due to 

 me, but to my friend Mr. Fox, F.R.M.S., from whom both 

 this pair of wedges and the next one are simply copied. 



Here is another preparation of the same character, built up 

 of twenty-four films, each of a thickness causing exactly \\ 

 of retardation for yellow light. Of course the thickness must 

 be very exact to bear multiplication twenty-four times without 

 sensible error. Now if we superpose on this wedge a flat 

 plate of mica with its axis* crossing that of the wedge, and of 

 a thickness equal to the middle stripe, that central stripe must 

 appear black when the Nicols are crossed (fig. 2), while New- 

 ton's first order of colours and half the second order appear 

 symmetrically on both sides of the black stripe. On one side 

 of the stripe the wedge itself gives greater successive thick- 

 nesses ; while on the other side the plate of mica does the 

 same. 



Such a wedge has further and real optical uses. It shows 

 at a glance the precise composition of every successively 

 increased \\ of retardation for the first three orders of New- 

 ton's colours. When extinction is complete for yellow light, 

 we know that a little of both red and blue must be unextin- 

 guished, the two giving us at the end of the first order the 

 opaque plum-colour known as the " tint of passage." As the 

 red at a given distance from the end of the spectrum is visu- 

 ally more conspicuous than the blue, at the end of the second 

 and third orders this " tint of passage " must become more and 

 more red, as we see on the screen is the case. The precise 

 composition of the light destroyed and that remaining, we may 

 demonstrate by placing a slit across the wedge and throwing 

 its spectrum on the screen (fig. 3), when we see the shifting of 

 the bands in steps for each \X of retardation. The wedge 

 alone shows only the first three orders ; but it is obvious that 

 by superposing a plate of mica 1 A, in thickness, the spectrum 



* Throughout this paper the " axis " of the mica is supposed to be that 

 one of its two polarizing planes which passes through the two optic axes. 



