222 Transactions of the Society, 



underlines, the letters are to all intents and purposes presented to 

 you as written. The various diagrams have been carefully copied 

 in facsimile for me by my colleague, Mr. B. K. Johnson. 



First Leiter. 



Jeka, 

 December 2nd, 1876. 



Dear Mr. Stephenson, 



Answering your kind letter. I begin with your question 

 about the diffraction-experiment. The grating in form of alter- 

 nately long and short lines (fig. 1) gives two sets of diffraction- 

 spectra, if it is put beneath the objective so that the line a . . . . 

 is in the middle of the field. The upper part will give a set of 

 spectra, the distance* of which is double the distance of the spectra 

 from the lower part, because the lines in the former part have just 

 half the distances. After adjusting the illuminating flame and 

 the position of the grating on the stage, you take out the eye-piece 

 and look down into the tube ; you see now the two sets of spectra, 

 one after the other, if you bring the pupil of the eye alternately 

 on the air-image of part A and of part B. The aspect is as 

 follows (fig. 2). Now you bring in the stop with three holes, 

 adjusted as in the figure, so that besides the more distant spectra 

 of both sets, the spectra m and n from B are stopped off, the three 

 central diffraction pencils of A remaining unaltered. Therefore 

 the image of part A will appear just as before, because the external 

 pencils are not necessary for forming an image. But as the effec- 

 tive pencils from part B now are made identical with those from 

 part A the image of B will be equal to the image of A ; it has less 

 brightness only Ijecause there is only half the light passing through 

 the lines of part B. 



You will get just the same effect with two holes (1 and 2) in 

 the diaphragm, because two different diffraction-pencils are quite 

 sufficient for forming an image, and two consecutive pencils in 

 every case give the normal image of the structure. 



I found unexpected difficulties in getting the silvered glass in 

 the right condition of the silver for ruling fine lines, and I was 

 obliged' to make many trials for that purpose, though formerly 

 there never occurred any difficulty. From that reason I liave got 

 such preparations as I showed you only just now : and you will 

 receive one in the next days, together with a contrivance for 

 turning the diaphragms above the objective witliout altering the 

 focus. You will find, on one slide, the grating (fig. 3) mentioned 

 above, and on both sides of it gratings crossed at right angle and 

 at the angle of 60° (figs. 4 and 6), just as I used wdien I had the 

 pleasure to show you the diffraction-experiments. The distance of 



* Abbe uses the word "distance " consistently for " distance apart." — F. C. 



