404 SLIPHER— NEBULA. 



spirals are sometimes found both types of spectra. This is true of 

 the nebulae numbered 598, 1068 and 5236 of the " New General 

 Catalogue" of nebulas. 



Some of the gaseous nebulas are relatively bright and their 

 spectra are especially so since their light is all concentrated in a 

 few bright spectral lines. These have been successfully observed 

 for a long time. Keeler in his well-known determination of the 

 velocities of thirteen gaseous nebulae was able to employ visually 

 more than twenty times the dispersion usable on the spiral nebulae. 



Spiral nebulae are intrinsically very faint. The amount of their 

 light admitted by the narrow slit of the spectrograph is only a small 

 fraction of the whole and when it is dispersed by the prism it 

 forms a continuous spectrum of extreme weakness. The faintness 

 of these spectra has discouraged their investigation until recent 

 years. It will be only emphasizing the fact that their faintness still 

 imposes a very serious obstacle to their spectrographic study when 

 it is pointed out, for example, that an excellent spectrogram of the 

 Virgo spiral N.G.C. 4594 secured with the great Mount Wilson re- 

 ilector by Pease was exposed eighty hours. 



A large telescope has some advantages in this work, but un- 

 fortunately no choice of telescope either of aperture or focal-length 

 will increase the brightness of the nebular surface. It is chiefly 

 influenced by the spectrograph whose camera alone practically de- 

 termines the efficiency of the whole equipment. The camera of the 

 Lowell spectrograph has a lens working at a speed ratio of about 

 1 : 2.5. The dispersion piece of the spectrograph has generally been 

 a 64° prism of dense glass, but for two of the nebulae a dispersion 

 of two 64° prisms was used. The spectrograph w^as attached to 

 the 24-inch refractor. 



With this equipment I have secured between forty and fifty 

 spectrograms of 25 spiral nebulae. The exposures are long — gen- 

 erally from twenty to forty hours. It is usual to continue the ex- 

 posure through several nights but occasionally it may run into weeks 

 owing to unfavorable weather or the telescope's use in other work. 

 Besides the exposures cannot be continued in the presence of bright 

 moonlight and this seriously retards the accumulation of observa- 

 tions. 



