166 REPORT—1868. 
This optical combination consists of a single prism of that kind which is used 
for direct vision, combined with a cylindrical lens. This combination allows 
us to employ the full light of the stars, not diminished as in common spec- 
troscopes by absorption, or by a slit and the several surfaces and thicknesses 
through which the light must pass. The image of the star in this system is 
formed in the focus as a luminous line of white colour if there is no prism; 
and with the prism the image is decomposed into a series of luminous lines 
arranged according to their refrangibilities, the interruptions due to the dis- 
continuity of the light appearing as black lines, 
In such a spectrum the relative position of the lines can be measured with 
a common screw-micrometer ; and their absolute position can be determined by 
comparison with fundamental stars, whose lines, on account of their in- 
tensity, can be fixed in an absolute manner relatively to known substances 
by a common slit-spectroscope. The comparison and measurement are 
rendered more easy by an improvement introduced in the instrument, by 
means of which I can see the direct image of the star together with its spec- 
trum. The superposition of this image on a spectral line in a part of the 
field of the telescope, marked by a wire, is susceptible of great nicety in 
measurement, and gives very accurate results. 
This, in a few words, was the apparatus employed in my researches, This 
year I have made a considerable improvement by employing an eyepiece 
made with cylindrical lenses only ; with these such an intensity of light is 
obtained that I have been able to observe the spectra of stars of the seventh 
and eighth magnitudes, which are of course quite invisible to the naked eye. 
Let us come now to the results. Many hundred stars of every magnitude 
to the sixth were passed in review. A catalogue of the chief of them has 
been made, and partly published. The work of the last year, yet unpub- 
lished, has been especially the examination of the red stars of smaller mag- 
nitudes, of which a particular research was instituted, but which was 
superseded after the reception of the catalogue of Prof. Schjelerup. All 
the objects contained in this catalogue (printed also in Chambers’s Treatise 
on Astronomy) have been examined to the eighth magnitude, beyond which 
limit my instrument cannot give a good spectrum. 
The principal results and conclusions at which I have arrived are these :— 
1st. All the stars in relation to their spectrum can be divided into four 
groups, for each of which the type of spectrum is quite different. 
The first type is represented by the stars Sirius, and Vega or a Lyre, and 
by all the white stars, as a Aquile, Regulus, Castor, the large stars in the 
Great Bear, a excepted, &c. The spectra of all these stars consist of an 
almost uniform prismatic series of colours, interrupted only by four very strong 
black lines. Of these black lines the one in the red is coincident with the 
solar line C of Fraunhofer ; another, in the blue, coincides with the line F; the 
other two are also in the sun’s spectrum, but they have no prominent place. 
These lines all belong to hydrogen gas; and the coincidence of these four 
black lines with those of the gas has been, by careful experiments, already 
proved by Mr. Huggins, and also lately by myself. In a Lyre the coinci- 
dence is found to be perfectly accurate. Mr. Huggins, however, finds a 
little difference in the spectrum of Sirius, for which we may account in an- 
other way, as I will explain presently. 
Stars of this first type are very numerous, and embrace almost one-half of — 
the visible stars of the heavens. We observe, however, some difference in 
individual stars; so that in some the lines are broader, and in others nar- 
rower; this may be due to the thickness of the stratum which has been 
traversed by the luminous rays. The more viyid stars have other very fine 
