422 Dr. H. Draper on Diffraction-Spectrum Photography. 



was carefully adjusted to the position of D 2 of the second order, 

 and the other to b A of the second order. When, therefore, after 

 a suitable exposure to the ultra-violet spectrum of the third order 

 the collodion picture was developed, there were two sharply 

 denned images of the steel points superposed on the spectrum. 

 The point which had been coincident with D 2 of the second 

 order was then found to have cast its shadow on H 2 of the third 

 order ; and the point at b 4 of the second order had impressed 

 itself near of the third order. 



By a simple calculation it was thus rendered evident that a 

 given ray in the compound line H 2 was of the wave-length 

 3930*1 ten-millionths of a millimetre, and that another line near 

 had the wave-length 3444*6. By looking at the photograph 

 the reader will see that 3930 falls upon a tine division in H 2 , 

 which is beautifully shown in both the spectrum with the scale 

 and the enlarged proof below. Of course the ray G of the third 

 order, the wave-length of which is known, had impressed itself 

 photographically on the collodion. 



Having thus ascertained the wave-lengths of three fixed points 

 in the photograph, the next step was to apply a scale reading to 

 a single ten-millionth of a millimetre, and, if possible, fractions 

 thereof. After many abortive attempts to use that part of 



Angstrom's map which lies between G and H, and to attach 

 thereto an additional length of scale sufficient to extend to the 

 end of the ultra-violet region, I was compelled to resort to a 

 linear dividing-engine, and rule a scale which was about twice 

 the length of the photographic reduction shown in the accom- 

 panying Plate. Of course this necessitated drawing in by hand 



the same systems of lines and lettering as are shown on Ang- 

 strom's chart ; and this I did as carefully and faithfully as I could. 



It only remained to reduce this divided scale to the proper 

 size to tit the spectrum -photograph; after many trials it was 

 accomplished. 



It is proper in this place to make a criticism on my scale and 

 to point out a small error, which may be due, however, to an 

 incorrect determination of the wave-lengths that I have used as 

 fixed points. Taking the distance from G (wave-length 4307) 

 in the photograph to the fixed line 3930 in H 2 and dividing it 

 into 377 parts, and then prolonging these divisions towards 0, 

 it was found that the third fixed point was not attained, but 

 that there was an error of about two divisions. But if the po- 

 sition of D 2 in Angstrom's determinations be incorrect to the 

 extent of one ten-millionth of a millimetre, or if this small 

 error should be partly attributed to D 2 and partly to G, my scale 

 will be correct. Future measures of the wave-lengths of these 

 rays and of b 4 can alone settle this delicate point ; for the deter- 



