This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1918 Excerpt: ...more readily seen. Astrophys. jfonrii., 19. 190 (1904). T. PC. 2 A CHAPTER X THE PRACTICAL RESOLVING POWER OF THE SPECTROSCOPE It was shown in Chapter III. page 74, et seq., how Lord Rayleigh deduced his well-known expression for the resolving power of a prism spectroscope. Lord Rayleigh found the relation where R is the resolving power, a the aperture, and--ah the dispersion of the prism train. By the resolving power is meant the ratio--, i.e. the ratio between the mean wavelength of a pair of lines that can just be resolved by the spectroscope and the difference in wave-length between the two components of the pair. It must be remembered that this refers strictly to an infinitely narrow slit, a condition that does not obtain in actual practice, and it is proposed to devote a short space to the consideration of the resolving power when the slit is given a definite width, as of course it must have under ordinary working conditions. It will be seen from what follows that the practical resolving power differs very considerably from the theoretical value with slit of infinitely narrow width, and since we are met with the fact that the giving of a finite width to the slit increases the amount of light available, and at the same time decreases the resolving power, we must, in designing any spectroscope, preserve the balance between the two best suited for the object in view. For diis reason it is hoped that the inclusion of the following discussion of the contributory factors may prove of service. For this discussion we are indebted to Schuster, who first dealt with the resolving power of a spectroscope with slit of a finite width. The problem was then taken up by Wadsworth, who dealt with Schuster's equation and modified it to a certain extent. This modificatio...