422 



NATURE 



\Scpt. 19, 1872 



collections kept for sale by him, and many colleges and other 

 cabinets in the United States contain series purchased from 

 him, furnished at very reasonable prices. 



The Ncwcastle-on-Tyne College of Physical Science has 

 issued its prospectus for the Session 1S72-73. The first Session 

 closed with 70 day and over 100 evening students, and consider- 

 able accessions are hoped for. The teaching of Biology is still 

 conspicuous by itsab5ence ; but there is so strong an element of 

 Natural History on the governing board of the College that we 

 may hope the just claims of this branch of science may not be 

 disregarded much longer. Why is the obsolete term "Natural 

 Philosophy" retained among the subjects taught? It is here 

 apparently meant to embrace Mechanics, Hydrostatics, and 

 Optics. An examination will be held on October 7 and 8 for 

 four exhibitions of 15/. each, tenable in the College for two years, 

 in Arithmetic, Algebra, and Euclid, and either Geology, Heat, 

 or Chemistry. 



In 1S71 the important papers of Dr. Petermann upon the 

 Gulf Stream, with their accompanying maps and charts, were 

 translated into English and puljlished by the United .States Ily- 

 drographic Office, under the direction of Captain K. H. Wyman. 

 Since then two supplements have been issued by the office, in- 

 cluding additional informition obtained by Dr. Petermann, the 

 second one accompanietl by a map of the northern regions of 

 Europe and Asia east of Greenland. This, which is on quite a 

 large scale, gives us the results of the discoveries made up to 

 the end of 1871, including the work done by Lamont, Mack, 

 Johannesen, Payer and Weyprecht, Rosenthal, &c. The text of 

 this supplement contains reports of the cruises of Smith and 

 Ulve, and of Captain Torkildser, papers on tlie sea north of 

 Spitzbergen, and on Gillis's Land and King Charles's Land, &c. 

 Petermann is of the opinion that, as far as the discoveries of 

 land go, the results of Smith and Ulve are more important than 

 those of any cruise between Greenland and Sil^eria for many 

 years past, as they show that the north-east line of Spitzbergen 

 extends across \o\ degrees of longitude instead of the 7A pre- 

 viously assigned, this extension including the southern co.ist 

 as well as the northern. The easternmost point reached by this 

 expedition was a little beyond the 2Sth degree of east longitude. 



At no previous period (says Harper's ]\'ivkly) has there been 

 so much activity displayed on the part of the United States 

 Government in the way of thorough explorations of its territories, 

 the liberality of Congress at the last session in authorising such 

 having been very great. The operations of the Coast Survey 

 have been largely extended, including the commencement of a 

 triangulation between the coasts of the Atlantic and Pacific. 

 Arrangements have been made for extended surveys by the Navy 

 Department of the North Pacific, and an appropriation also made 

 for the expense of making the observation of the coming transit 

 of Venus. Under the War Department are progressing the new 

 survey of the northern boundary of the United States, between 

 the Rocky Mountains and the Lake of the Woods, the geological 

 survey of Mr. Clarence King along the fortieth parallel, and the 

 surveys in Utah and Nevada I>y Lieutenant George M. Wheeler ; 

 while Dr. Hayden's work in the Interior Department is advanc- 

 ing satisfactorily in its two main divisions, as also that of Major 

 Powell along the Colorado. 



There are at this time four chemical laboratories in Japan, 

 where the science is taught, three of them being presided over 

 by Germans and the fourth by an American. The chief one is 

 at 0;aka, where there are nearly 100 students. The rest are at 

 Kaga, Shidzoka, and Fukuwi. A fifth will soon be opene.l at 

 Jeddo. The students are said to be fairly intelligent, but their 

 minds are at present encumbered with astrology and other kinds 

 of spurious philosophy. 



THE BRITISH ASSOCIATION 



SECTION A— M,iTHEM.\Tic.\L AND Phy.sical Science 



On the Application of Photography to Copy Di fraction 

 Gratings, by the Hon. J. W. Strutt. 



Gre.^t interest has always attached itself to the beau' ful 

 phenomena discovered by Frauenhofer, which present themselves 

 when a beam of light falls on a surface ruled with a great numb-jr 

 of parallel and equidistant lines. Their unexpected character, 

 the brilliant show of colour, and the ready explanation of thi 

 main point on the principles of the wave theory, recommend 

 them to all ; while the working physicist recognises in them the 

 key to the exact measurement of wave lengths, which has been 

 so splendidly used by Angstroni and others. 



The producnon, however, of gratings of sufficient fineness and 

 legularity is a matter of no ordinary difficu!fv. Indeed, the 

 exactness recpiired and obtained is almost incredible. The wave- 

 lengths of the soda lines differ by about the thousandth part. If 

 in two gratings, n\- two parts of the same grating, the average 

 interval betvveen the divisions differed by this fraction, the less 

 refrangible soda line of one would bo superposed on the more 

 refrangible corresponding to the other. In point of fact the 

 gratings ruled by Nobert, to whom the scientific world 

 has been greatly indebted, are capable of distinguishing a dif- 

 ference of wave-length probably of a tenth part of that above 

 mentioned. Hut in order that the D lines may be resolved at all, 

 there must be no average error (running over a large part of the 

 grating) of , r,',-, rr part of the interval between consecutive lines. 

 When it is remembered what the interval is (from tAtt to ttsVo 

 of an inch, or even less), the degree of success which has been 

 reached seems very remarkable. 



A work requiring so much accuracy is necessarily costly — the 

 reason, probably, why gratings fit to be used with the telescope 

 for the purpose of showing the fixed lines are comparatively rare. 

 The hope of being able to perfect a process for the reproduction 

 of gratings at a comparatively cheap rate has induced me to return 

 at the first opportunity to the experiments described in a prelimi- 

 nary note read before the Royal Society in June last. Although 

 the subject is as yet by no means exhausted, I have thought it 

 worth while to Ining before the Section an account of the pro- 

 gress that has been made, with specimens of the results. 



The method of procedure is very simple, h. dry plate pre- 

 pared by any photographic process on a flat surface of glass or 

 other transparent material not affected by the fiuid media 

 employed, is brought into contact with tlie ruled surfaces of the 

 grating in a printing frame, and exposed to light. In my first 

 experiments I used exclusively as a source of light the image of 

 the sun in a lens of short focus placed in the shutter of a dark- 

 ened room ; but so small a source is not necessary. The light 

 from the clouds or sky, reflected by a mirror through a hole 

 several inches in aperture, will be sufficiently concentrated if the 

 frame be a few feet distant. I have not as yet specially investi- 

 gated the point, but I believe that if the light were too much dif- 

 fused the experiment would fail. Much would, no doubt, depend 

 on the perfection of the contact — an element very likely to vary. 



The variable intensity of diffused daylight, which it is almost 

 impossible to estimate with precision, has induced me to use ex- 

 clusively in my later experiments with ordinary photographic 

 plates the light of a moderator lamp. This, with globe removed, 

 is placed at a distance of one or two feet from the printing frame, 

 the distance being carefully measured. Working in this way 

 there is little difficulty in giving consecutive plates any relative 

 exposures that may be required. A collateral advantage is the 

 possibility of operating at any time of the day or night. 



With regard to the preparation of the plates, I have latterly 

 been using the tannin process introduced by Major Russell. A 

 preliminary coating with dilute alliumen is generally advisable, 

 as any loosening of the film from the glass must be avoided, on 

 account of the distortion that it might introduce. In some slates 

 of the collodion, an edging of black varnish put on after the 

 exposure is sufficient to hold the film down. Tlie glasses, after 

 being coated with collodion (I have used Mawson's), are im- 

 mersed as usual in the silver bath, and then allowed to soak in 

 distilled water, best contained in a dipping-bath. They are then 

 washed under a tap for about half a minute, and put into the 

 tannin solution (about I5grains to the ounce) held, in my prac- 

 tice, in a small dish. I usually prepare my plates in. the evening, 

 standing them up to dry on blotting-paper. In the morning 

 they are in a fit state for use. Artificial heat might no doubt be 

 used if a more rapid drying were desired, 



