CHAPTER X. 



MISCELLANEOUS RESULTS OF THE PRECEDING EXPERIMENTS. 



79. Spectrum phenomena due to moving motes. In connection with this 

 work I incidentally came upon a curious phenomenon which seemed to repay 

 special investigation. To describe it, it will be advan- 

 tageous to first indicate the disposition of apparatus 

 used, as is done in figure 103. Here L is a pencil of 

 white light, preferably from a collimator, impinging on 

 the thin cylindrical glass shell G about 10 cm. in diam- o 

 eter and containing a solution of mercury potassic 

 iodide about half an inch deep and not quite concen- 

 trated. The rays are thus both refracted and dis- 

 persed, and on emerging enter the strong objective of 

 a short-range telescope (magnification above 15) of 

 which PP is the principal plane and rb the narrow 



spectrum seen in the ocular of the telescope. Properly focussing the latter, 

 the spectrum may be contracted to a vividly colored vertical line. 



If now a strong direct vision grating g is inserted in front of the objective, 

 and the telescope is focussed anew, a sharp solar spectrum may be obtained. 

 This was a surprise to me, as the cylinder G, though thin and clear, was ob- 

 tained from samples of ordinary glass shades, such as are prized by the lover 

 of stuffed birds. In other words, the cylinder supplies its own slit, as at r 

 or b in the figure, by refraction. With a narrow beam of sunlight no colli- 

 mator is needed. 



The spectrum will now be found to be filled with short, slender, horizontal 

 shadows, all moving endwise in a common direction, but at different speeds. 

 On pushing the ocular in somewhat further, these shadows become sharply 

 defined lines, all nearly horizontal, of all lengths, from mere points to black 

 lines half the length of the spectrum or more. On attentive observation the 

 black lines are seen to be associated with narrow areas of accentuated bright- 

 ness, so that diffraction patterns are in question. Occasionally a beautifully 

 complete, slender, spindle-shaped black body with a brilliant narrow frame of 

 light around it will be seen. Arrow-heads holding patches of light on their 

 notched sides are not infrequent, but as a whole the spectrum appears 

 to be intersected with an interminable array of horizontally flying arrows, 

 all shot in a common direction from end to end. With regard to the motion, 

 this is (more usually) horizontally from red to blue ; in the middle layers and 

 in the lapse of time always so and not permanently from blue to red. Some- 

 times both motions were seen to occur together in different levels, the retro- 

 grade motion being relatively slow, less pervasive, and confined to the top 

 or the bottom layers. All degrees of speed occurred from a passage through 

 the spectrum in a fraction of a second, to passage lasting over a minute . Under 

 104 



