July 4, 1889] 



NATURE 



227 



adopted, in which, by means of a check list at the 

 end, it is shown in what American libraries all the 

 periodicals may be found. A good portion of the work 

 is done in such works as have been mentioned : in the 

 indexes to the literature of special subjects in the Smith- 

 sonian collections, in the publications of the Harvard 

 University, and in the indexes to the publications of 

 Societies, such as the Chemical, Geological, Zoological, 

 Linnean, Astronomical, Geographical, Engineers, Statis- 

 tical, the Society of Arts, and the Royal Society, and in 

 the indexes of various periodicals. These should be 

 systematically arranged ; the chaos should be organized 

 and classified, to enable the man of science to find out at 

 a glance all that has been published on any branch of 

 his subject, and the work would be of value to the 

 country. 



The range of subject-headings should include every- 

 thing relating to scientific and technical subjects. These 

 would include (taking Comte's classification for con- 

 venience in the serial arrangement). Mathematics, Astro- 

 nomy, Physics, Chemistry, Physiology (or what is com- 

 monly understood as Natural History, with the generic 

 term of Physiology or Physio-Philosophy), and Social 

 Physics (including Sociology). These subjects are clear, 

 well defined, and well known to the librarians. 



There is nothing so necessary as that scientific thought 

 and method should be embodied, classified, and arranged, 

 preliminary to its organization as a whole. It would 

 quicken the slow process of improvement that has 

 extended over a thousand years. It is wonderful that it 

 should be necessary to say this in the nineteenth century. 

 The need for organization in all departments of science is 

 keenly felt ; and the growth of Positivism in these latter 

 days is one of the expressions of that need. Three 

 centuries ago Bacon aimed at the organization of the 

 sciences, holding that the sciences can be advanced only 

 by combining them ; that, as natural laws are invariable 

 and uniform, " Physics being the mother of all science," 

 so moral and civil philosophy could not flourish when 

 separated from their roots in natural philosophy. 



On national grounds it is necessary that this work 

 should be done, for it is in the interest of the community 

 generally, of the nation, that we should know what we 

 possess. Public libraries and educational institutions are 

 increasing ; scientific experiment, discovery, and inven- 

 tion are increasing ; and the demand for such a subject- 

 catalogue will increase. Its value to the community 

 would be inestimable. If it is thought advisable that the 

 Royal Society or the Society of Arts should take the 

 work in hand, the work should be subsidized by the 

 Government, for the simple reason that it would be of 

 national value. Scientific research is not so remunerative 

 but that the student may fairly expect facility of access to 

 the labours of those who have gone before. The want of 

 a serviceable key to the vast body of scientific work con- 

 tained in our literature is daily experienced by cultivators 

 of science. There is a serious necessity that this material 

 should be made more readily accessible for comparison, 

 for verification, and for improvement. Much of it is a 

 monument of shattered hopes, the unsuccessful efforts of 

 poverty and despair ; but all of it is suggestive to the 

 earnest student. 



No one Society or publisher can take the work in hand 

 without Government support or benefaction. Attempts 

 have been made, such as the attempt of the Royal 

 Society, and that of Agassiz in his " Bibliographia Zoolo- 

 gies et Geologifc," published by the Ray Society, which 

 voluminous work, be it remembered, " was mainly com- 

 piled by the Professor for his own private use during 

 the leisure moments of a life of almost incessant scientific 

 research." In whatever hands the work may be placed, 

 there is no doubt that the co-operation of the scientific 

 Societies might be relied upon. With regard to the 

 question of assistance from the Government, the following 



Minute of the Lords Commissioners of Her Majesty's 

 Treasury, dated November 28, 1864, referring to the 

 Royal Society's Catalogue, is very suggestive : — 



" Having regard to the importance of the work with 

 reference to the promotion of scientific knowledge gene- 

 rally, to the high authority of the source from which it 

 comes, and to the labour gratuitously given by members 

 of the Royal Society to its production, my Lords consider 

 themselves justified in having the work printed at the 

 cost of the public, with the understanding that, reserving 

 such a number of copies for presentation as my Lords, in 

 communication with the President of the Royal Society, 

 may hereafter determine, the work shall be sold at such a 

 price as may be calculated will repay the cost of printing. 



" Their Lordships, however, desire it to be understood 

 that the work shall go forth to the public under the 

 authority of the Royal Society, by the exertions of whose 

 members this important aid to the study of science has 

 been produced." J. Taylor Kay. 



IRIDESCENT CRYSTALS.^ 



'TPHE principal subject of the lecture is the peculiar 

 ■*■ coloured reflection observed in certain specimens of 

 chlorate of potash. Reflection implies a high degree of 

 discontinuity. In some cases, as in decomposed glass, 

 and probably in opals, the discontinuity is due to the 

 interposition of layers of air ; but, as was proved by 

 Stokes, in the case of chlorate crystals the discontinuity 

 is that known as twinning. The seat of the colour is a 

 very thin layer in the interior of the crystal and parallel 

 to its faces. 



The following laws were discovered by Stokes : — 

 (i) If one of the crystalline plates be turned round in 

 its own plane, without alteration of the angle of incidence, 

 the peculiar reflection vanishes twice in a revolution, viz. 

 when the plane of incidence coincides with the plane of 

 symmetry of the crystal. [Shown.] 



(2) As the angle of incidence is increased the reflected 

 light becomes brighter and rises in refrangibility. [Shown.] 



(3) The colours are not due to absorption, the trans- 

 mitted light being strictly complementary to the reflected. 



(4) The coloured light is not polarized. It is produced 

 indifferently whether the incident light be common light 

 or light polarized in any plane, and is seen whether the 

 reflected light be viewed directly or through a Nicol's 

 prism turned in any way. [Shown.] 



(5) The spectrum of the reflected light is frequently 

 found to consist almost entirely of a comparatively narrow 

 band. When the angle of incidence is increased, the 

 band moves in the direction of increasing refrangibility, 

 and at the same time increases rapidly in width. In 

 many cases the reflection appears to be almost total. 



In order to project these phenomena a crystal is pre- 

 pared by cementing a smooth face to a strip of glass, 

 whose sides are not quite parallel. The white reflection 

 from the anterior face of the glass can then be separated 

 from the real subject of the experiment. 



A very remarkable feature in the reflected light remains 

 to be noticed. If the angle of incidence be small, and if 

 the incident light be polarized in or perpendicularly to the 

 plane of incidence, the reflected light is polarized in the 

 opponic manner. [Shown.] 



Similar phenomena, except that the reflection is white, 

 are exhibited by crystals prepared in a manner described 

 by Madan. If the crystal be heated beyond a certain 

 point the peculiar reflection disappears, but returns upon 

 cooling. [Shown.] 



In all these cases there can be little doubt that the re- 

 flection takes place at twin surfaces, the theory of such 

 reflection {Phil. Mag., Sept. 1888) reproducing with re- 



' Abstractor the Friday evening lecture delivered by Lord Rayleigh, F.R.S. 

 at the Royal Institution on April 12, 1889. 



