3i8 



NA TURE 



[February 3, 1898 



The nearest approach to the production of the stereoscopic 

 eflfect from a single picture is by photographing the illustration 

 with its plane inclined at an angle of the optic axis of the lens, 

 first in one direction, and then again in the opposite direction ; 

 in which manner two dissimilar pictures are produced, which 

 can be viewed with an ordinary stereoscope, and the result 

 is by no means unsatisfactory. The process, however, is 

 inconvenient. 



For a long time past the writer has been experimenting with 

 stereoscopy, and amongst other objects one has been to secure 

 a stereoscopic effect with a single picture. This he has now 

 succeeded in attaining in a comparatively simple manner. 



If a stereoscopic slide is examined it will be found that each 

 picture is compressed, so to speak, on the inner side, which is 

 equivalent to saying that the central line of any one picture is 

 shifted from the centre inwards, thus expanding one half of the 

 picture and compressing the other half. 



It therefore appeared to the writer that, if by some means a 

 single picture could be made to give two images, each one com- 

 pressed in a suitable manner and then viewed with the ordinary 

 stereoscopic lenses, the solid effect should be produced, and such 

 is the case. 



The method is as follows : — A large deep cylindrical lens is 

 taken and cut in two along the line where the lens is thinnest. 

 Thus two wedges are produced, each having one side curved 

 and the other side flat. If these two pieces are placed together, 

 with the thick portions towards one another, and held at a short 

 distance over any picture, the eyes will perceive an image of the 

 illustration in each lens, and the pictures will be compressed in 

 the manner already referred to. 



All that is now necessary is to view these two images with 

 ordinary stereoscopic lenses. One picture will result, as solid 

 in appearance as if the ordinary stereo slide had been employed. 



The apparatus will be found of considerable use for looking 

 at small photographic portraits, landscapes, &c. , as well as 

 engravings. The result is very pretty. 



Naturally one class of picture must suffer under the operation ; 

 but as these are comparatively rare, there is an extended use for 

 the instrument, which is called the Pseudoscope. 



The pictures unsuitable are those which represent an object 

 when placed very near the eyes, since the image produced by 

 solid objects in each eye in such cases differ greatly ; but this 

 difference in the image is comparatively small after a distance of, 

 say, 15 to 20 feet. David Salomons, 



MAGNETIC OBSERVATIONS IN THE HARZ 

 MOUNTAINS. 



T^R. ESCHENHAGEN has made an interesting series 

 -L^ of magnetic observations at forty-two stations in 

 the Harz Mountains. 



The uncertainty of a declination observation was 

 about i', of the dip with two needles about i''2, and 

 the probable error of the horizontal force was about 

 ± o'oooio C.G.S. units. As it was intended to compare 

 the results with those of a survey of the same district in 

 which the deviations of the plumb-line were determined, 

 the stations were selected with reference to this fact, and 

 also to the geological conformation of the country. 



The magnetic disturbances and disturbing forces were 

 determined by a method essentially similar to that 

 employed in the survey of the United Kingdom. The 

 results were not very definite in the northern part of the 

 district examined, but a clearly marked " ridge-line," or 

 locus of attraction on the north-pole of the magnet, was 

 discovered in the south. In one part of this the vertical 

 disturbing force nearly reached 0*00400 C.G.S. units. 

 There are few stations in England and Wales where the 

 disturbance is as large as this, though at Stratford-on- 

 Avon it is exceeded by 25 per cent. At some places 

 purely local attractions of very much larger magnitude 

 were detected. Thus, on the Leistklippe the dip was 

 altered by 11', and the vertical disturbing force was 

 0*04224 C.G.S., or one-tenth of the whole vertical force. 

 No such remarkable effects were observed near the 

 principal granitic masses, such as the Brocken. 



The fact that the magnetic ridge-line lies to the south 



NO. 1475, VOL. 57] 



of these is explained by the hypothesis that the granitic 

 mass, of which the Brocken is the upper end, extends 

 underground in a direction inclined to the vertical, 

 and running from the surface towards the south. 

 This view is, in the opinion of Dr. Eschenhagen, con- 

 sistent with the geological facts. As, however, the 

 granite is but slightly magnetic, it is also necessary to 

 suppose that the heavier, more magnetic granite, lies 

 deep, and is only brought near to the surface at a 

 few points such as those at which the greatest local 

 disturbances occur. 



The local deviations of the plumb-line were deduced 

 from observations made by the Royal Prussian Geodetic 

 Institute. To the north of the mountains the plumb-line 

 is deflected to the south ; from the south an attraction to- 

 wards the north is detected. The line of no deviation is to 

 the south of the Brocken, and runs nearly parallel to, 

 but about 10-12 km. north of, the magnetic ridge-line. 



The remarkable result is therefore attained that (i) 

 geological evidence, (2) the magnetic needle, and (3) the 

 plumb-line all point to the conclusion that the heavier 

 rocks lie some distance to the south of the chief visible 

 masses of granite. 



Dr. Eschenhagen is careful to point out that it is un- 

 likely that a similar agreement would be attained in every 

 place where similar experiments might be carried out, as 

 the rocks of greatest density are not necessarily magnetic. 

 None the less, his observations make it probable that 

 the magnet, the pendulum and the plun)b-line may add 

 much to our knowledge of the details of the constitution 

 of the crust of the earth ; and his survey of the Harz 

 Mountains is one of the most striking attempts which 

 have yet been made to combine the results of magnetic 

 and geodetic surveys. A. W. R. 



THE JOURNAL OF ANATOMY AND 

 PHYSIOLOGY. 



NEARLY nine years ago, on the occasion of the 

 coming of age of the above-named Journal, we 

 directed attention (Nature, vol. xxxvii. p. 441) to its as- 

 sociation, as a medium of publication, with the then 

 newly established " Anatomical Society of Great Britain 

 and Ireland." Since that time the connection between 

 the two has been maintained, much to the advantage of 

 the Journal, for not only have the " Proceedings " of the 

 Society materially improved and increased in bulk 

 and importance, but many of the leading papers which 

 have appeared in the body of the Journal have reached 

 it through the mediation of the Society, while in the 

 latter's annual reports of its " Committee of Collec- 

 tive Investigation" there have been contributed results 

 of great service alike to the scientific and surgical 

 anatomist. Owing to the death of the founder of the 

 Journal and main supporter during its earlier years, the 

 beloved and universally respected Sir G. Humphry, a re- 

 constitution of the staff of its " conductors " was recently 

 decided upon, and with the issue for October last a new 

 series was accordingly commenced, Profs. Sir W. Turner 

 and J. G. McKendrick, so long associated with its suc- 

 cess, announcing that for the future they would be 

 assisted by Profs. D. J. Cunningham, A. Macalister, and 

 G. D. Thane. There has just reached us the second 

 (January) part which has appeared under these new 

 auspices. During the years which have elapsed since 

 our previous notice was written, three of the five present 

 conductors of the Journal have been presidents of the 

 Anatomical Society, while Prof Macalister, having been 

 on November 26 elected to that office, now combines 

 the two functions. And during the same period the 

 Anatomical Society undertook the preparation, and the 

 conductors of the Journal the publication, of a couple of 

 analytical indexes to its first thirty volumes, entrusting 

 the task to Mr. A. W. Kappel, the efficient librarian of 



