176 Intelligence and Miscellaneous Articles. 



ON A CONVENIENT EYEPIECE-MICROMETER FOR THE SPECTRO- 

 SCOPE. BY PROFESSOR O. N. ROOD. 



I have recently contrived a very simple eyepiece-micrometer for 

 the study of spectra produced by prisms and " gratings," which, 

 while quite inexpensive, is capable of yielding results that are not 

 easily surpassed except by the use of an eyepiece provided with a 

 micrometer-screw. A thin semicircular plate of silver is made 

 quite smooth, and rendered black by holding it over the flame of a 

 lamp ; it is afterward flowed with a drop of weak spirit-varnish to 

 cause the lampblack to adhere. Crossing the straight edge of this 

 dead-black surface, lines \ millim. &c. are ruled with a dividing- 

 engine, and the necessary figures added with the help of a lens. 

 The opaque semicircular plate is then introduced into the interior 

 of a negative, or preferably in front of a positive eyepiece, so that 

 it is in focus and does not occupy quite half of the field of view. 

 Opposite it and somewhat nearer the eye an opening is made in 

 the side of the eyepiece, whereby the lines are brightly illuminated 

 — as a general thing, merely by the diffused light of the room ; but 

 if this is quite dark, the small flame of a distant lamp easily accom- 

 plishes the same end. This arrangement, it will be seen, furnishes 

 a set of bright lines on an almost perfectly black ground with the 

 least possible outlay of expense or trouble in manipulation ; and the 

 degree of their brightness, it will be found, can readily be regulated 

 merely by shading the opening more or less with the hand. The 

 distance of the lines apart should not be too small, as is often the 

 case in the photographic scales of ordinary spectroscopes, but such 

 as will facilitate the estimation of tenths of a division. Two such 

 eyepieces have been constructed and employed by me with much 

 satisfaction in the mapping of a large number of spectra furnished 

 by prisms and gratings, particularly in those cases where the spec- 

 tral lines were quite faint. — Silliman's American Journal, July 

 1873. 



jamin's compound magnets. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 Is not the method of making powerful compound magnets pub- 

 lished by M. Jamin substantially identical with that employed by 

 Scoresby, and fully set forth in his ' Magnetical Investigations ' so 

 long since as 1839 ? He shows that thin hard steel plates may be 

 superposed on the other with great advantage if separated by thin 

 slips of wood or cardboard, and states that the power of such plates 

 accumulated efficiently "to the amount of 192, and might have 

 been carried much further." 



I am, &c, 

 General Post Office, E. S. Ctjlley. 



June 25, 1873. 





