no 



For the total amount of light received is the same, hence the 

 brilliancy of the band will be inversely as its length. But 

 for bright lines proceeding from an incandescent vapour, the 

 same inverse proportion betvsreen brilliancy and dispersion does 

 not hold. Let us take the case of hydrogen, one of the chief 

 constituents of the solar atmosphere. By an increase in the 

 number of prisms, all we effect in their case is a separation of 

 the four bright images of the slit or Hues from one another by a 

 greater interval, without very appreciably dimming their light. 

 Hence if we wish to view the prominence spectrum, it will be 

 necessary first to cover the image of the sun by the slit-plate and 

 so cut off the direct rays, then to increase the number of prisms, 

 removing by this means the continuous spectrum caused by 

 reflected rays, and the bright lines sought for will be obtained. 



Having now obtained the prominence spectrum, it only remains 

 to travel round the limb of the sun, and whenever the bright hne 

 of hydrogen is seen rising up beyond the ordinary level of the 

 chromosphere, we know that we are viewing a prominence. It 

 will not be out of place to describe the method adopted in this 

 observation at Stonyhurst. The spectroscope is carried by a tail- 

 piece which is securely clamped to the eye-end of the telescope. 

 This tail-piece has a graduated circle which can be read to sec- 

 onds of arc, and an attached vernier can be rotated round it at 

 the same time as the spectroscope, by means of a rack and pinion. 

 Therefore in the first place it is only necessary to make the zero 

 division of the scale exactly coincident with the apparent N. point 

 of the sun, and the index of the vernier when rotated will tell to 

 what part of the limb, as measured from the N. point, we are 

 directing our gaze. But more than this, by another simple ad- 

 justment the spectroscope may be so rotated round the sun's disc 

 that the slit may be kept either in a tangential or a radial position 

 to the limb. At the eye-end of the small viewing telescope, 

 placed after the prisms, is a micrometer, by which the height of 

 the line of light may be read off. In this way it is possible to 

 view a flame in successive, almost vertical sections, and putting 

 the readings together, a very fair picture can be drawn. And so 

 we can travel round the sun and observe all the prominences to 

 be seen on any given day. Or if the tangential slit be preferred, 

 successive horizontal slices of any prominence can be taken, and 

 the flame depicted. Though the forms can be obtained by a re- 

 duction of the measui'es made, yet the work is long and tedious, 

 and it would be obviously far better to study these objects by view- 

 ing them as a whole. This was the next problem proposed for 

 solution to the skill of spectroscopists. 



Lockyer and Jaussen proposed to use the one an oscillating slit, 

 the other a rotating slit adjusted to a direct-vision spectroscope, 

 the idea being that a succession of sectional images coming rapidly 



