240 Spectroscope Apparatus 



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

 its length must be proportioned to the size of prism, and con- 

 sequently 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 disad- 

 vantage of losing light, and being deficient in separating 

 power. So important is the size and focal length of the tele- 

 scope, 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 collmiating tele- 

 scope determines 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 recommend and apply telescopes of 

 much greater focal length than had hitherto been used in spec- 

 troscope 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 

 adjustment, will show the colours brighter, the lines sharper 

 and much more numerous, and will separate or iC resolve" more 

 of the compound lines. It will also give an intelligible spec- 

 trum, 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 requires, 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, as in the apparatus of Mr. Browning, 

 shown in the annexed cut, is amply sufficient. With this instru- 

 ment the entire spectrum can be displayed at once "with a magni- 

 fication of 30 or 40, and a lower positive eye-piece, conveniently 

 furnished with cross wires, enables all the principal 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 vastly better than those which Mr, Browning made at an 

 earlier period on Air. Crookcs' pattern, with two prisms, and 

 telescopes of smaller size. 



We may take this new instrument as the best in quality 

 and form that has been produced to meet ordinary requirements ; 

 and, for common purposes, a moro elaborate and complicated 

 apparatus is not necessary, and, indeed, would uot always be ad- 

 vantageous. The eye soon gets accustomed to the appearances 



