Ox COXCAVE GRATINGS FOR OPTICAL PURPOSES 503 



We have numerous cases of wide lines, such as the C line, the compon- 

 ents of the D 3 and H lines and numerous others which are perfectly 

 familiar to every spectroscopist. Hence we are free to suppose that all 

 lines have some physical width, and we are limited by that width in the 

 resolving power of our spectroscope. Indeed, from a theoretical stand- 

 point, we should suppose this to be true : for the molecules only vibrate 

 freely while swinging through their free path and in order to have the 

 physical width one one-hundred-thousandth of the wave-length, the 

 molecule must make somewhat nearly one hundred thousand vibrations 

 in its free path: but this would require a free path of about sooVoo 

 inch ! Hence it would be only the outermost solar atmosphere that could 

 produce such fine lines and we could hardly expect to see much finer 

 ones in the solar spectrum. Again * it is found impossible to obtain 

 interference between two rays whose paths differ by much more than 

 50,000 wave-lengths. 



All the methods of determining the limits seem to point to about the 

 150,000th of the wave-length as the smallest distance at which the two 

 lines can be separated in the solar spectrum by even a spectroscope of 

 infinite power. As we can now nearly approach this limit I am strongly 

 of the opinion that we have nearly reached the limit of resolving power, 

 and that we can never hope to see very many more lines in the spectrum 

 than can be seen at present, either by means of prisms or gratings. 



It is not to be supposed, however, that the average wave-length of the 

 line is not more definite than this, for we can easily point the cross- 

 hairs to the centre of the line to perhaps 1 in 1,000,000 of the wave- 

 length. The most exact method of detecting the coincidences of a line 

 of metal with one in the solar spectrum would thus be to take micro- 

 metric measurements first on one and then on the other; but I suppose 

 it would take several readings to make the determination to 1 in 

 1,000,000. 



Since writing the above I have greatly improved my apparatus and 

 can now photograph 150 lines between the H and K lines, including 

 many whose wave-length does not differ more than 1 in about 80,000. 

 I have also photographed the 1474 and b 3 and & 4 , widely double, and also 

 E just perceptibly double. With the eye much more can be seen, but 

 I must say that I have not yet seen many signs of reaching a limit. The 



3 1 have recently discovered that each component of the D line is double probably 

 from the partial reversal of the line as we nearly always see it in the flame spectrum. 



*This method of determining the limit has been suggested to me by Prof. C. 8. 

 Hastings, of this University. 



