490 HENRY A. EOWLAND 



can also focus on the superimposed spectrum and so get the focus for 

 the portion experimented on. 



The fact that the light has to pass through no glass in the concave 

 grating makes it important in the examination of the extremities of 

 the spectrum where the glass might absorb very much. 



There is one important research in which the concave grating in its 

 present form does not seem to be of much use, and that is in the exami- 

 nation of the solar protuberances; an instrument can only be used for 

 this purpose in which the dust in the slit and the lines of the spectrum 

 are in focus at once. It might be possible to introduce a cylindrical 

 lens in such a way as to obviate this difficulty. But for other work on 

 the sun the concave grating will be found very useful. But its principal 

 use will be to get the relative wave lengths of the lines of the spectrum, 

 and so to map the spectrum; to divide lines of the spectrum which are 

 very near together, and so to see as much as possible of the spectrum; 

 to photograph the spectrum so that it shall be normal; to investigate 

 the portions of the spectrum beyond the range of vision; and lastly to 

 put in the hands of any physicist at a moderate cost such a powerful 

 instrument as could only hitherto be purchased by wealthy individuals 

 or institutions. 



To give further information of what can be done in the way of grat- 

 ings I will state the following particulars : 



The dividing engine can rule a space 6| inches long and 4 inches 

 wide. The lines, which can be 4^ inches long, do not depart from a 

 straight line so much as nnnnnr inch, and the carriage moves forward in 

 an equally straight line. The screw is practically perfect and has been 

 tested to nnmnj" inch without showing error. Neither does it have any 

 appreciable periodic error, and the periodic error due to the mounting 

 and graduated head can be entirely eliminated by a suitable attachment. 

 For showing the production of ghosts by a periodic error, such an error 

 can be introduced to any reasonable amount. Every grating made by 

 the machine is a good one, dividing the 1474 line with ease, but some 

 are better than others. Eutherfurd's machine only made one in every 

 four good, and only one in a long time which might be called first-class. 

 One division of the head of the screw makes 14,438 lines to the inch. 

 Any fraction of this number in which the numerator is not greater 

 than say 20 or 30 can be ruled. Some exact numbers to the millimetre, 

 such as 400, 800, 1200, etc., can also be ruled. For the finest definition 

 either 14,438 or 28,876 lines to the inch are recommended, the first for 

 ordinary use and the second for examining the extremities of the 



