Proceedings of the Polytechnic Association. 1013 



of vision, and in this respect is a remarkable adaptation of furce to 

 precisely the object to be attained. In all devices for illumination 

 by artificial means, many waves, moving both slower and faster than 

 those required, are simultaneously generated, so that but a small 

 portion of the radiant energy exerted is made available to human 

 vision. Yet it may be doubted whether any strong and serviceable 

 light can be produced without a simultaneous generation of the whole 

 range of undulations from the slowest to the most rapid, as found in 

 solar rays. 



Eeversion Specteoscope. 

 Dr. J. C. F. Zollner, of Leipsic, has explained before the Royal 

 Society of Saxony his new .form of spectroscope, by which spectra 

 are reversed. In his arrangement the line of light produced by a 

 slit, or cylindrical lens, lies in the focus of a lens, which in all spec- 

 troscopes renders parallel the rays to be dispersed. Then the rays 

 pass through two direct-vision prism-systems, so fastened to one 

 another that each passes one-half of the pencil of rays proceeding 

 from the collimator object-glass, and, also, so that the refracting 

 angles lie on opposite sides. In this way the collected pencil of rays 

 will be dispersed in the two spectra in an opposite direction. The 

 object-glass of the observing telescope, which unites the rays again 

 to an image, is perpendicular to the refracting angles of the prisms 

 placed horizontally, and, as in the heliometer, is divided ; each of the 

 two halves can be moved micrometrically both parallel to the line of 

 section and perpendicular to it. By means of this the lines of one 

 spectrum can be bi'ouglit into coincidence with those of the other, 

 and the spectra can be placed in immediate juxtaposition, instead of 

 being superposed, so that one spectrum moves by the other like a 

 vernier, or the spectra may be superposed only partially. In this 

 construction not only is the delicate principle of double images 

 rendered available for the determination of any change whatever 

 in the position of the lines of the spectrum, but any such change is 

 doubled, since its influence appears in the two spectra in an opposite 

 sense. To use the instrument without the system of prisms it is 

 only Jiecessary to reverse one part of the pencil of rays proceeding 

 fi'om a common prism by reflection on a mirror or prism, and then 

 to observ^e the united pencil of rays exactly as before described with 

 a telescope furnished with a divided object-glass. The simultaneous 

 introduction of artificial sources of light for the investigation of 



