CELESTIAL CHEMISTRY. 203 



waistcoat-pocket, will serve to indicate the presence of the most char- 

 acteristic substances, or to show to the traveler whether the atmos- 

 phere as is often the case of an afternoon and in thunder-storm 

 weather develops any lines of moderate magnitude that are not reg- 

 ularly seen. A small spectroscope, however, labors under two obvi- 

 ous limitations : Its prism cannot usefully receive much light; open- 

 ing the slit too wide introduces confusion, as just explained, and its 

 length must be proportioned to the size of prism, and consequently 

 be restricted when a little one is employed. Then the telescopes 

 attached to a small spectroscope are of small aperture and short focal 

 length, with the consequent disadvantage of losing light, and being 

 deficient in separating power. So important is the size and focal 

 length of the telescope, that an apparatus of four prisms and three-feet 

 telescopes exhibits more lines than one of nine prisms furnished with 

 two-feet telescopes. The focal length of the collimating telescope de- 

 termines the apparent size of the slit. If it is short, and the curves of 

 its glass considerable, the slit looks wider, and all the lines are spread. 

 This action when noticed by Mr. Browning, induced him to recom- 

 mend and apply telescopes of much greater focal length than had hith- 

 erto been used in spectroscope apparatus. The first requisites for a 

 better examination of spectra will then consist in a larger prism, with 

 telescopes of larger aperture and greater focal length. They collect 

 more light, and are better able to separate closely adjacent lines. 

 The larger apparatus, if equally good in its corrections and adjust- 

 ment, will show the colors brighter, the lines sharper and much more 

 numerous, and will separate or "resolve" more of the compound 

 lines. It will also give an intelligible spectrum, with a quantity of 

 light that a smaller instrument could not work with. 



For the traveller and casual observer the small pocket form answers 

 well. The chemist and physicist require, for many purposes, more 

 light, a wider dispersion, and a greater resolving or separating power. 

 For ordinary laboratory use, none of these properties are needed in 

 excess ; and a single fine prism, with two large telescopes, is amply 

 sufficient. With this instrument the entire spectrum can be displayed 

 at once with a magnification of 30 or 40, and a lower positive eye- 

 piece, conveniently furnished with cross wires, enables all the princi- 

 pal lines in the darker part of the spectrum, as well as in the more 

 luminous, to be seen distinctly, and their position noted down. This 

 spectroscope, with one prism of 60, and telescopes of an inch and a 

 quarter clear aperture, and about 18 inches focal length, is decidedly 

 the best in quality and form that has been produced to meet ordinary 

 requirements ; and, for common purposes, a more elaborate and com- 

 plicated apparatus is not necessary, and, indeed, would not always be 

 advantageous. The eye soon g'ets accustomed to the appearances of 

 different spectra, and a momentary glance would suiKce to show 

 whether sodium, potassium, lithium, silver, etc., were absent or pres- 

 ent. But it is necessary that the exact position of the lines or bands 

 should be ascertained, and that means should be provided by which 

 exact diagrams may be made. Hitherto only superior instruments, 

 beyond the reach of ordinary students, have been furnished with these 

 desiderata; but Mr. Browning, an optician of London, has now sup- 

 plied them, in a convenient form, and at a very low price. One of 



