472 Prof. PL A. Rowland on the Results accomplished 



we have a most beautiful method of determining the relative 

 wave-lengths of the different portions of the spectrum, which 

 far exceeds in accuracy any other method yet devised. In 

 working in the ultra-violet or ultra-red portions of the 

 spectrum, we 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 examination 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 photo- 

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

 the portions of the spectrum beyond the range of vision ; and, 

 lastly, to put into 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 gratings 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 \\ inches long, do 

 not depart from a straight line so much as tooVou i ncB > an( l 

 the carriage moves forward in an equally straight line. The 

 screw is practically perfect, and has been tested to tg^q-q 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 reason- 

 able amount. Every grating made by the machine is a good 

 one, dividing the 1474 line with ease; but some are better 

 than others. Rutherford'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 



