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NOBERT'S TESTS AND MR. WEBB. 



The following letter has been received from Dr. Woodward : — 



Army Medical Museum, 



Washington, D.C., August 18th, 1873. 



To the Members of the Quekeit Microscopical Club. 



Gentlemen, — As I always read the Journal of your Club with 

 interest, my attention was at once arrested by the communication 

 of Mr. William Webb '' on Nobert's T«.sts," in the July number, 

 in which he arrives at the conclusion " That beyond the first few 

 bands of Nobert's Tests there is not one containing a line properly 

 so called." The mechanical considerations urged by Mr. Webb I 

 will not discuss further than to say that he appears to have over- 

 looked completely one of the most striking facts with regard to 

 Nobert's plates, viz. : That the lines of the first band are not only 

 further apart, but are more deeply ruled than those of the second ; 

 that those of the third are still shallower, and so on progressively. 

 This circumstance, it appears to me, destroys his whole argument. 

 I do not, however, write to discuss Mr. Webb's argument, but to 

 remind the members of the club that there is a physical reason 

 which compels us to believe that the first fifteen bands, at least, of 

 the nineteen band plate are composed of real and distinct lines, and 

 that the distance of these lines apart must approximate very closely 

 to what was intended by Nobert. 



When the bands of the Nobert's plate are illuminated by oblique 

 light, and are looked at from above with a low power (too low to 

 show any of the lines), each band appears as a smooth^coloured 

 stripe. From the known wave length of the colour^seen, and the 

 angle of the incident pencil, the distance which the lines of any 

 band must actually be apart can be computed by the well-known 

 formula for the spectrum of gratings enunciated by Fraunhofer, 

 and the distance thus obtained agrees with_^ that ^at which Nobert 

 ruled the lines. On the other hand the angle of the incident pencil 

 being known, and Nobert's given distance being assumed to be 



