Spectroscope Apparatus. 241 



of different spectra, and a momentary glance would suffice to 

 show whether sodium, potassium, lithium, silver, etc., were 

 absent or present. 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 has now supplied them, in the instrument figured, 

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

 telescopes in the instrument we are describing moves over a 

 graduated 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 ascer- 

 tained. 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, 

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

 Browning has carried out with his accustomed skill. 



This instrument is composed of a cylinder capable of being 

 rotated, and fixed at any point. It cairies a graduated scale 

 exactly corresponding 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 accord- 

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

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

 diagrams to be made with precision, and a hundred observers 

 in different parts of the world, 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 propor- 

 tionate delicacy. 



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

 in order that their discrepancy or conformity may be ascertained 

 by mere inspection. This was accomplished first, we believe, 

 by Frauenhofer. In Mr. Browning's instrument it 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 



