264: ANNUAL OF SCIENTIFIC DISCOVERY. 



the telescopes in the instrument we are describing moves over a grad- 

 uated arc, read off to one minute by a vernier. Any line taken as a 

 starting-point is brought into the centre of the field, so that it forms a 

 perpendicular, cutting through the centre of the cross wires with 

 which one eye-piece is furnished. The telescope is then clamped and 

 the vernier read. Other lines are taken in succession, and their exact 

 angular distance ascertained. When only moderate accuracy is 

 needed in a diagram, a piece of paper can be ruled to a scale, and 

 the lines laid down accordingly. Great nicety is not, however, to be 

 obtained in this way ; and Mr. Beckley, of the Kew Observatory, has 

 suggested the plan of a " spectrograph," which Mr. Browning has 

 carried out with great skill. 



This instrument is composed of a cylinder capable of being rotated, 

 and fixed at any point. It carries a graduated scale exactly corre- 

 sponding with the scale of the spectroscope, so that when the cross 

 wires of the latter instrument intersect a line, which when read off on 

 .the vernier stands at, say 10, the cylinder can be adjusted so that a 

 delicate metal ruler indicates the exact spot on which a line should be 

 drawn on a slip of paper which the cylinder carries. If the next line 

 to be mapped down is 5 distant from the first, the cylinder is moved 

 accordingly, the ruler is again in its exact place, and the second line 

 is drawn as correctly as the first. This instrument thus enables dia- 

 grams to be made with precision, and a hundred observers in different 

 parts of the Avorld, furnished with spectroscopes and spectrographs, 

 properly graduated, would be certain of producing diagrams capable 

 of the exact comparison that science needs. The spectroscopes that 

 can measure fractions of a minute can be furnished with spectrographs 

 of proportionate delicacy. % 



It is often advisable to have two spectra in the field at once in or- 

 der that their discrepancy or conformity may be ascertained by mere 

 inspection. This is effected in a very simple way. Half the slit of 

 the spectroscope is permitted to receive light from a source directly 

 in front of it. The other half of the slit is covered by a small right- 

 angled prism which prevents direct light getting in, but reilects rays 

 that reach it at right-angles to the axis of the instrument. 



For special purposes it is necessary to view and measure the posi- 

 tion of the more delicate lines which the spectrum contains, and to 

 separate some which look single with ordinary means. It would at 

 first seem that the way to do this would be to employ more magnifica- 

 tion, to treat, in fact, the close lines as we do those on diatoms, and 

 separate them by magnifying power. Practically this plan only ad- 

 mits of very restricted application. Two plans are adopted to get 

 over the difficulty, the spectrum is made wider by greater dispersion, 

 our fan of light is spread out more ; and telescopes of large aper- 

 ture and greater focal length furnish the spectroscopist with a greater 

 resolving or separating poiver, the increase of magnification being 

 produced by the object-glasses instead of the eye-pieces. 



The reader will appreciate the difference between magnifying a 

 spectrum of given dispersion, by remembering our illustration of the 

 fan. Let a fan be opened, so that a portion of the pattern painted 

 on its spokes is concealed by overlapping. It is obvious that magni- 

 fying the fan in this state can give us no information concerning the 



