Notes and Memoranda. 401 



NOTES AND MEMORANDA.. 



The Dabe: Past of Vests' Disc. — Dr. Engelmann, of the Leipzig Obser- 

 vatory, speaking of successful trials made with a new 8-inch glass of Steinheil, 

 states that on the 20th of April the dark side of Yenus was very strikingly seen. 

 It appeared of a clear greenish-grey tint. 



Xetv Spectboscopes. — The Herschel-Browning spectroscope, which has now 

 been some months before the public, is recognized as the best instrument of its 

 class, viz., that of direct vision spectroscopes having considerable dispersion, and 

 obtaining the direct vision with little loss of light. It is composed of two prisms, 

 each possessing direct vision, in consequence of the light ray experiencing two 

 internal reflexions. As the principles involved in the construction of these 

 instruments are of considerable interest, and can only be made clear by elaborate 

 explanation and diagrams, Mr. Alexander Herschel has, at our request, supplied 

 a paper on the subject, which will shortly appear, and we shall now only exhibit 

 the form of the spectroscope devised by himself, and modified by Mr. Browning. 

 We have taken several opportunities of looking through these spectroscopes, and 

 have been much struck by the sharpness with which they exhibit, with a low- 

 power, a great multitude of dark lines. For quick examination of atmospheric 

 lines, and for noting the changes that occur near the horizon, or in any particular 

 direction, this form of spectroscope i3 the best yet devised, as it can be instan- 

 taneously and accurately pointed at any cloud or in any direction. Its dispersion 

 and precision are so great as to divide the line D with a magnifying power of 

 only 5. 



In the Herschel-Browning spectroscope considerable dispersion is one of the 

 objects sought and obtained ; but, as we explained in our last number, there are 

 occasions on which the dispersion should be comparatively small, and the instru- 

 ment adapted to work with a minimum of light. Mr. Browning exhibited at the 

 Royal Society's soiree an instrument of this kind, made for Mr. Gassiot. It con- 

 sists of three prisms, united together by Canada balsam, to prevent loss of light 

 by reflection at the surfaces of contact. The centre prism is of dense glass, and 

 the two side ones of lighter glass, so adjusted as to counteract the refraction of 

 the centre prism, and leave two-thirds of its dispersive power. We tried this 

 instrument in various ways with much gratification. From its exquisite defi- 

 nition it exhibits the Frauenhofer lines in greater quantity and with more sepa- 

 ration* than would have been expected, and with a careful management of light 

 shows the blood absorption bands in extremely weak solutions. The solution we 

 employed was obtained by macerating a very small morsel of raw meat in an ounce 

 of coid water. A few drops of this solution, in about an ounce of water, gave a 

 noticeable result, and we did not by any means arrive at the extreme limits of the 

 power of the instrument. We must postpone a description of two other new 

 spectroscopes exhibited by Mr. Browning at the Royal Society's soiree ; one is the 

 " Bigid Spectroscope" made for Mr. Gassiot, "to test whether any relationship 

 exists between refraction and gravitation," and the other a striking optical instru- 

 ment of recreation, " the Revolving Spectroscope, producing changing symmetrical 

 figures of gorgeous colours in endless variety." 



* The apparent separation depends partly on the amount of dispersion, and partly on per- 

 fection of definition. A good prism, like a good telescope, will show a division where a worse 

 instrument with the same power would ehow coalescence. 



