Chemistry and Physics. 473 



an auxiliary screen at a short distance from the incidence side 

 of the collimator slit. This screen, of course, is divided by a 

 cut the edges of which are parallel to, but farther apart than, 

 the corresponding edges of the neighboring slit. For photo- 

 graphing the radiations from the chromosphere and promi- 

 nences at the sun's limb the direct light from the solar disk is 

 cut out by an occulting disk having a diameter slightly less than 

 that of the image formed by the telescope objective. This occult- 

 ing screen is situated in front of the slit screen and it is moved 

 at exactly the same rate as is imparted to the solar image by the 

 declination motor. 



After the light emerges from the collimator lens as a parallel 

 beam it experiences one reflection at a plane mirror and then 

 passes through a train of two prisms at minimum deviation. 

 The total deviation is 180° so that the optic axes of the colli- 

 mator and camera systems are parallel, and the entire spectro- 

 heliograph assumes the compact form of the letter U. On 

 emerging from the second prism the various wave-lengths of the 

 dispersed beam of light are brought to their respective foci in 

 the focal surface of the camera lens. The second or camera slit 

 is unilateral, it can be rotated around an axis normal to the 

 plane of its jaws, and it can be translated in a plane perpen- 

 dicular to the principal axis of the camera objective so as to 

 allow one, and only one, narrow spectral line to pass through 

 it to the photographic plate. If the first slit were straight and 

 the second slit were given the necessary curvature (arising from 

 oblique refraction through the prism train) the image of the 

 sun would be greatly distorted, flattened on one side and drawn 

 out on the other. This distortion is entirely eliminated by 

 employing lenses of the same focal length and by making the 

 radii of curvature of the collimator and camera slits twice as 

 great as would be the radius of curvature of the images corre- 

 sponding to a straight first slit. The direction and speed of 

 translation are the same for both the photographic plate and the 

 solar image on the first slit. When the dark (Fraunhofer) 

 solar lines are used it is necessary to increase the resolving power 

 of the dispersing system. This is accomplished by suitably 

 adjusting a plane speculum grating between the mirror and the 

 first prism. The grating employed has 20,000 lines to the inch 

 on a ruled surface 2% by 3% inches. For studying the flocculi 

 the H and K lines of calcium are usually found to yield the best 

 results. Certain lines due to hydrogen, the radiations A4226-9 

 Ca and A4383-7 Fe, and a few others, are sometimes used to 

 advantage. 



In Part I, the detailed description of the Eumford spectro- 

 heliograph is appropriately followed by a preliminary account 

 of the results obtained with this instrument. The discussion 

 relates to the nature and minute structure of calcium flocculi, 



Am. Jour. Sci.— Fourth Series, Vol. XLV, No. 270.— June, 1918. 

 33 



