Nov. 25, 1875] 



NATURE 



63 



of expression is welcome, but not at the expense of 

 accuracy. 



Then, again, we wonder what was the object of the 

 translator in tacking on to many words their German 

 equivalent ; if the meaning were doubtful to him, or 

 could not be rendered, that would be all very well ; but 

 when a plain man finds in the text the statement that 



Fig. I.— Refraction and internal reflection in a raindrop. 



solid bodies are held together " by a powerful force which 

 is termed the force of cohesion (Zusammenhangkraft)," he 

 is apt to be a httle frightened. The translator possibly 

 ' ' that after the insignificance of our English term, it was 



essar)' to give a powerful German word to express the 

 erful force. This, however, does not explain the 



urrence of German expressions elsewhere in brackets, 



These little defects are perhaps incidental to the first 

 ...ion of a translation ; of Prof. Lommel's work we have 

 already expressed cur high opinion. The explanations of 

 ' 1 enomena are extremely clear and precise, and here 



i there appendices furnish elementary mathematical 

 reasonings which, though wholly omitted from the text, 

 are desirable in some places to give the reader a more 

 complete knowledge. The chapters in which the undu- 



FiG. 2, — Spectra of crown and flint glass. 



latory theory is employed to explain the phenomena of 

 reflection, refraction, &c., seem to us extremely useful and 

 clearly written. Nowhere, in an elementarj' book, have 

 |we met with so simple and elegant an explanation of the 

 reason why the energy of a vibration is proportional to 

 the square of its amplitude as that given by the author 

 on pp. 227 and 228. 



Here is a simple experiment to illustrate the formation 

 of the rainbow, that we have not before seeji in a text- 

 book : — 



" Upon a glass sphere k, filled with water and having a 

 diameter of four centim. (i| in.), a beam of solar light of 

 equal or greater diameter than the sphere is allowed to 

 strike horizontally, and there is then seen, upon a large 

 screen ss, placed in front of the sphere, and per- 

 forated in its centre to allow the passage of the incident 

 rays, arranged concentrically to the aperture and at a 

 distance from it which is nearly equal to that of the sphere 

 from the screen, a beautifully coloured circle, in fact a 

 circular spectrum, the colours of which are arranged con- 

 centrically, and in such a manner that the red is outside 

 and the violet on the inside. At a still greater distance 

 from the centre of the screen a second similar but much 

 fainter circle is observed, the colours of which however 

 succeeded one another in the inverse order, the red appear- 

 ing on the inside, and the violet at the outer periphery." 

 (p. 122.) 



The different dispersive power of bodies is instructively 

 shown by comparing the spectra given by crown and 

 flint glass (Fig. 2), wherein it is seen that although the total 

 dispersion, that is the length of the spectrum, is exactly 

 the same, the mode of dispersion is different. By the 

 position of the Fraunhofer lines in the two spectra, " it is 

 rendered evident that the less refrangible rays are more 

 closely approximated in passing through the flint glass, 

 whilst the more refrangible are separated further from one 

 another than by the crown glass." (p. 139.) 



The difference in the nature of the dispersion is 

 subsequently sho^vn (chapter xviii.) to be caused by 

 a difference in the rate of propagation of the various 

 undulations when passing through many solids and 

 liquids. Hence "the proposition that all kinds of 



Fig. 3 — Unusual dispersive power of Fuchsin. 



light are propagated with equal rapidity, which was 

 shown to be true of the free ether of the universe, 

 is found to be no longer admissible for the ether 

 contained in the interior and occupying the inter- 

 stices of the particles of natural substances." Very 

 strikingly is the influence of the nature of the material 

 particles on the velocity of propag ation exhibited in those sub- 

 stances in whose absorption spectra one or more very dark 

 lines appear. " If we introduce, for example, an alcoholic 

 solution of the aniline colour 'fuchsin,' into a hollowprism, 

 and look through it at a brightly illuminated sUt, we 

 obtain a spectrum in which blue and violet are less de- 

 flected than yellow and red. What is elsewhere the end 

 of the spectrum here appears at the commencement ; 

 towards the middle it fades, and in the centre the green, 

 being absorbed, is absent (Fig. 3). From this behaviour the 

 conclusion may be drawn that in 'fuchsin' the blue and 

 violet rays are propagated with greater velocity than the 

 red and yellow. This phenomenon, which was dis- 

 covered by Christiansen, and was shown by Kundt to be 

 presented by a great number of absorbing substances, 

 has been called the anomalous dispersion of lights 

 (p. 244.) 



