86 



formance is due to want of absolute unifonnity in the grating 

 space. This is clue to the enormous difficulty in constructing 

 a screw which shall be practically perfect throughout its whole 

 length — a difficulty which increases very rapidly as the length 

 of the screw increases, and it has been supposed that the limit 

 of accuracy was reached in these gratings. 



The great and rapidly increasing importance of spectrum 

 analysis, especially in determining the distribution of light in 

 so-called spectral lines under normal conditions, in the resolution 

 of complicated systems of lines, and in the investigation of the 

 effects of temperature, of pressure, and especially of a magnetic 

 field, justified the undertaking of much larger gratings than 

 these. As an example of progress made in this direction, I have 

 the honor of exhibiting a grating having a ruled surface nine 

 inches long by four and one-half inches stroke (220mm>< iiomm.) 

 This has one hundred and ten thousand lines and is nearly per- 

 fect in the second order, so that its resolving power is theoretic- 

 ally 220,000, and is very nearly realized in actual experiments. 



It will be observed that the effect produced at the focus of 

 the telescope depends on the concurrence or opposition^ — in gen- 

 eral on the interference — of the elementary trains of light-waves. 

 We are again indebted to the genius of Newton for the first 

 observation of such interference; and a comparatively slight 

 modification of the celebrated experiment of "Newton's rings" 

 leads to a third method of spectrum analysis which, if more in- 

 direct and less convenient than the methods just described, is 

 far more powerful. If two plane surfaces (say the inner sur- 

 faces of two glass plates) are adjusted very accurately to 

 parallelism, and sodium light fall on the combination at nearlv 

 normal incidence, the light reflected from the two surfaces will 

 mterfere, showing a series of concentric rings alternately bright 



