ASTRONOMY AND METEOROLOGY. 307 



the last of the train, to which the right-angled reflecting prism is 

 cemented. 



" Placing this first half prism with its front face perpendicular to 

 the line of collimation, it would never need to be disturbed ; the 

 flange of the cylindrical frame which carries the prisms would be 

 firmh' fastened to the bed-plate immediately beneath it, and the 

 pivot" joint at this place with the corresponding tangent-screw 

 would be dispensed with. The only adjustment required would 

 be that produced by the screw which is now used to adjust 

 for minimum deviation by opening or closing the gap of the 



Of course, this arrangement would reduce the dispersive power 

 of the train by the amount of one prism, a loss easily made up by 

 adding a degree or two to their refracting angles. 



" It might be better to place the face of the first prism not exactly 

 normal to the line of collimation, in order to avoid repeated re- 

 flections between it and the object-glass of the collimator, which 

 would be likely to produce a troublesome ghost, or the same thing 

 might be accomplished by simply cementing the object-glasses of 

 both collimator and ob serving-telescope directly upon the front 

 of the prism ; this would make the instrument still more solid and 

 compact. 



" The eye-piece of the instrument has an apparatus attached, 

 which, however, thanks to the high dispersive power, I find 

 unnecessary. 



" It was early proposed by Janssen to use a vibrating or rotating 

 slit, in order to make visible the form of a solar prominence, but, 

 as Zb'llner has shown, the mere opening of the slit answers just as 

 well, the light of the protuberance being diluted to precisely the 

 same extent in either case. 



"It occurred to me in connection with a suggestion of Professor 

 Morton, that, by interposing at the focus of the eye-piece a dia- 

 phragm which should move with the vibrating slit, the light of the 

 neighboring portions of the spectrum might be cut oif and this di- 

 lution avoided. Mr. Clark has devised and constructed a very 

 beautiful mechanical arrangement by which this simultaneous and 

 accordant motion of slit and diaphragm is effected by the rotation 

 of a small fly-wheel. 



*' But I find that, although seen in this way, the prominences 

 appear very bright; yet the working of the apparatus always 

 causes a slight oscillation of the equatorial, which interferes with 

 the definition of details, and I prefer to work with the slit simply 

 opened. When the air is free from haze, the whole extent of a 

 prominence 30,000 miles in height is readily examined through 

 the C or F line, and the most delicate details reveal themselves, 

 with a beauty and clearness of definition which even yet always 

 surprises me, and speaks most emphatically for the exquisite 

 workmanship of the 43 different surfaces by which the light is 

 either refracted or reflected on its way from the slit of the colli- 

 mator to the eye. 



"But, although I do not use the vibration of the slit and dia- 

 phragm, I find the mobility of the slit so convenient as to be 



