Spectroscope Apparatus. 245 



The different spectroscopes to which we have alluded are 

 all constructed upon the principle of indirect vision. That is 

 to say, the observer does not point the telescope straight at 

 the light he wishes to see. It reaches him round the corner 

 through the refraction of the prism. This plan is handy enough 

 for fixed apparatus, but for rapid examination of the light that 

 comes from different portions of the sky, or from radiating ob- 

 jects, a spectroscope of direct vision is preferable. Mr. Brown- 

 ing effects this by placing a dense flint glass prism of 60° be- 

 tween two prisms of light crown of 22°. Other opticians follow 

 the same principle, with variations of detail. Such spectro- 

 scopes act clearly, but they lose much of the dispersive power 

 of the chief prism. In Mr. Browning's pattern the loss is 

 about one-third. Although employed by Secchi and other 

 eminent observers, spectroscopes of this kind are by no means 

 the best for astronomical research; though amateurs who merely 

 desire to see the principal lines which bright celestial objects 

 afford, will probably find them adapted to their purpose. 



Although we must not now anticipate the paper on Mr. 

 Huggins' recent discoveries, which we shall shortly publish, 

 we may describe the arrangement with which he arrived at the 

 wonderful fact that the spectra of certain nebulae resemble 

 those afforded by highly rarefied and intensely heated gases. 

 The rays from the object-glass of his telescope are received by a 

 cylindrical lens of half-inch focus, on emerging from which 

 they enter the slit of the collimator. The spectroscope has 

 two prisms, and is furnished with a micrometer screw. This 

 apparatus was constructed by Mr. Browning. 



One of the most interesting branches of spectroscope in- 

 quiry is the absorption of certain portions of the spectrum by 

 solutions. The fluid can be put in a test tube ; but for many 

 of these experiments a prismatic cell is better. This consists 

 of a rectangular glass cell, one side of which is composed of 

 a prism. When the solution to be examined is put in the cell, 

 it forms a fluid prism ; and if the glass prism and the solution 

 prism correspond pretty closely in refractive power, one undoes 

 the refractive work of the other, and thus light is permitted to 

 come straight through the combination to the slit of the colli- 

 mator. 



To view the spectra of gases, narrow tubes are employed, 

 in which the gas, in a highly rarefied state, is rendered incan- 

 descent by the discharge of a RuhmkorfPs coil. 



In all spectroscope experiments, the edges of the slit of the 

 collimator should be kept very clean. Dirt particles, or rust, give 

 rise to vexatious black lines traversing the spectrum from end 

 to end. Mr. Browning informs us that they are best removed by 

 the gentle action of a thin wedge cut out of hard slate pencil. 



