MATHEMATICS AND NATURAL PHILOSOPHY. 455 



yellow glass and covered with it one half of this plate. I then exposed the whole 

 to the direct rays of the sun for ten minutes, and afterwards pi need the same 

 plate in the camera in order to attempt to take a picture upon it. In developing, 

 the part I had covered with yellow glass after previously exposing the whole to 

 the light, produced a picture, (though not a very good one), and the part left un- 

 covered, produced, as might have been expected, a perfect mass of blacL ness ; hence 

 I conclude that the yellow glass undid the work that had previously been done by 

 the ordinary light." 



Photographers seem now to be pretty generally convinced that positive paper 

 prints, if toned by hyposulphite of soda, will fade when exposed to moisture. Mr. 

 Shadbolt publishes a process in which sulphide of silver (a permanent compound) 

 is substituted for the ordinary sulphuret. The paper is salted on a bath of gela- 

 tine, 1 gr. ; Chloride of Ammonium, 10 grs ; Water, 1 oz. After exposure, it is 

 washed with water, then with liq. amm. fort, diluted with four or five times its 

 bulk of water ; again washed with plain water, and then toned with a solution of 

 hydrosulphite of ammonia ; a final washing and drying completes the picture. 

 The tone is said to be an agreeable brownish black, which acquires a yellowish 

 tinge by time. 



THE GREENWICH OBSERVATORY. 



In the report of the Astronomer Royal to the Board of Visitors, the following 

 curious fact is mentioned : " There is a well-marked annual periodical change in 

 the position of the Transit Circle, the southerly movement of the eastern pivot 

 having its minimum value in September, and its maximum in March, the extreme 

 range being about 14 seconds ; and there is a similar change, but of smaller 

 amount, in the position of the Collimator. I cannot conjecture any cause for these 

 changes, except in the, motion of the ground. There is a very frequent change of 

 still smaller amount in the Azimuth of the Transit Circle, accompanied by a 

 nearly equal change in the apparent Azimuth of the Collimator, so that from 

 day to day the Transit Circle and Collimalor preserve their relative position un- 

 altered; these I conceive to be the effects of accideut in observation of the circum- 

 polar stars, arising either from fault of the observer, or from irregularities either 

 in the level or in the collimation ; at the same time, viewing the great accuracy of 

 the observations of circumpolar stars, and the extreme simplicity of the pivot- 

 supports and of the instrument frame, I cannot conjecture how such irregularities 

 can arise." During the past winter, Mr. Airy received intimation from Prof. 

 Hansteen that the dip, as determined at Greenwich, appeared to have become 

 greater than was consistent with the changes of dip going on in the North of Europe. 

 A similar discordance was found to exist between Greenwich and Kew. This led 

 Mr. Airy to examine the observatory instrument, and it was found so imperfect in 

 its mechanical construction, that when the needle was lifted up from its agate bear- 

 ings, its upper point almost always struck the brass circle. These defects have been 

 amended, and the apparent dip is diminished by nearly the quantity which Prof. 

 Hansteen conjectured. Mr. Airy regrets that this irregularity unfortunately causes 

 the dip-observations at Greenwich for several years past to possess very little value. 



COLONIAL MAGNETIC OBSERVATORIES. — BY MAJOR, GEN. SABINE. (PROC. R. S.) 



The magnetic investigations designed to be carried into execution by the Co- 

 lonial Observatories recommended by the Royal Society, embraced a much wider 



