May 23, 1878] 



NATURE 



107 



Arabia, presented by Commander F. Catton ; a Wood Owl 

 {Syrnium aluco), European, presented by Mr. C. B, Wharton ; 

 a Copper Head Snake {Ctnchris contortrix) from North Ame- 

 rica, presented by Dr, Painter; a European Bearded Vulture 

 {jGypaetus barbatus). South European, a Rattlesnake (Crotalus 

 durissus) from North America, deposited ; a Collared Fruit 

 Bat (Cynonyderis collaru), a Reindeer {Ran^ifer tarandus), a 

 Chinchilla {Chinchilla lanigera), born in the Gardens. 



RECENT RESEARCHES ON THE PHENOMENA 

 OF FLUORESCENCE 



COME time ago Herr E. Lommel drew attention to the fact that 

 ^ certain substances do not follow the rule mentioned by Prof. 

 Stokes, viz., that each ray of light produces fluorescent rays of 

 smaller refrangibility only ; recently Herr Lommel found several 

 other substances which partly follow and partly deviate from 

 Stokes's rule, which is thus proved to be of somewhat limited 

 ralidity. Herr Lommel communicated his further researches on 

 this subject at a recent meeting of the Physical Society of 

 Erlangen. He now divides all fluorescent bodies into three 

 classes : the first class comprises those substances upon which 

 each homogeneous ray of light, capable of producing fluorescence, 

 produces the whole fluorescent spectrum (fluorescence of the 

 first order) ; the second cla -s contains those substances upon which 

 the same ray of light produces only those rays of the fluorescent 

 si3ectrum which are of a smaller (or at most of an equal) refrangi- 

 bility than the ray itself (fluorescence of the second order). 

 The third class finally, embraces those bodies whose fluorescent 

 spectrum consists of two parts, one of which corresponds to 

 fluorescence of the first, and the other to fluorescence of the 

 second order (compound fluorescence). Herr Lommel enume- 

 rates nine substances of class I., twenty-five of class H., and 

 seven of class III., and gives the commencement of their 

 fluorescence in the spectrum and the extent of the latterin tables 

 accompanying his paper. 



If we examine the scPies of substances enumerated by Herr 

 Lommel, we find no less than fifteen different ones, which deviate 

 from Stokes's rule, i.e., depart from it altogether (the nine 

 substances of class I.) or only in part (the first six substances of 

 class III.). Of course the substances in class II. which follow 

 this rule are more numerous. Investigating the peculiarities of 

 these three classes more closely, and in various directions, we 

 arrive at the following conclusions : — 



I. The first class comprises substances with very'strong bands 

 of absorption only, of which one remains visible even when the 

 solution is greatly diluted and after the absorption in other parts 

 of the spectrum has become imperceptible. Accordingly these 

 substances are strongly and intensely coloured (green, red, 

 orange, yellow). The absolute maximum of fluore.xence 

 corresponds to this absolute maxxmum of absorption in the 

 fluorescence-spectrum. 



2. The second class embraces all fluorescent substances which 

 show only a one-sided absorption of the more refrangible end of 

 tie spectrum. They therefore appear yellow, brown, or colour- 

 less, the latter principally if the absorption bands lie only in the 

 extreme violet and ultraviolet. 



To this class certain bodies belong. which 'show absorption 

 bands to which maxima of fluorescence correspond at the same 

 time. But these absorption bands appear broad and indistinct 

 (so-called shadows) and they are not really maxima. If the 

 solution is diluted they disappear very soon, and long before 

 the absorption in the violet ceases to be perceptible. Nitrate 

 of uranium shows a number of strongly marked absorption 

 bands, which, however, are in nowise related to fluorescence. 

 Uranium glass as well, which belongs to the first class, shows 

 absorption bands in the red and yellow which have nothing 

 to do with its fluorescence. The green colour of the fluor spar 

 from Alston Moor is caused by an absorption band in the red, 

 and this also has no reference to fluorescence. 



3. The third class, like the first, contains only bodies with 

 strong absorption bands and of intense coloration (green, blue, 

 violet, red, orange). 



Fluorescence of the first order therefore seems to be in causal 

 connection with the existence of a prominent maximum of 

 absorption and fluorescence. 



4. The fluorescent spectrum of the substances in class I. is of 

 •equal colour everywhere, if we neglect the slight changes in the 



shades of colour, which are caused by the absorption exercised 

 by the substance upon its own fluorescent light. At the parts which 

 are less fluorescent, a proportionate diminution of the fluorescent 

 rays most subject to absorption takes place, and hence a 

 deepening of the shade ; these rays are at the same time the 

 more refrangible ones in all bodies belonging to this class. 



5. The fluorescent spectrum of the substances in class 11. 

 is of unequal colour, changing gradually its tone of coloiu", and 

 it only becomes equal where the spectrum of the fluorescent 

 light ends. But even here the fluorescent spectrum may seem 

 of equal colour everywhere to the naked eye. This is the case 

 where the spectra of the exciting and the excited rays are super- 

 posed on one another only a little (morine-alumina solution ; nitrate 

 of uranium), or if the fluorescence begins in the blue or in 

 the violet. 



6. The fluorescent spectrum of the bodies in class III. con- 

 sists of two parts, viz., one equally coloured in its whole length 

 (less refrangible), and one coloured differently, with gradual 

 change of colour (more refrangible), becoming equally coloured 

 only where the total spectrum of the fluorescent lights ends. 

 At the boundary of the two parts an almost sudden change of 

 colour takes place. 



7. The substances of class III. behave like mixtures of a 

 substance of class I. and one of class II. 



The solutions of orchil and litmus appear as mixtures of 

 two fluorescent substances, from the fact that, according to the 

 dissolving medium, now one and now the other fluorescence 

 is prominent, and the aspect of the total fluorescence changed. 

 From two different kinds of litmus Herr Lommel obtained 

 alcoholic extracts, of which the one showed orange and the 

 other greenish-yellow fluorescence in daylight ; their fluor- 

 escent spectra, however, showed no difference, except that the 

 yellowish-green part was proportionately more developed in the 

 second specimen. Fluoride of aniline may also be considered 

 as a mixture of two fluorescent substances. That brasileine is 

 a mixture of this kind is rendered very probable by the circum- 

 stance that the fluorescence of the second order is destroyed by 

 the addition of soda, but strengthened by the addition of ammo- 

 nia, while that of the first order remains intact. Whether the 

 chamaeleine colouring matters are really chemical compounds, as 

 they certainly appear to be, must first be decided by a closer 

 investigation of these substances. 



If it could be proved beyond doubt that all these substances 

 are mixtures of two fluorescent substance?, we might abolish 

 class III. altogether, and enumerate the separated substances in 

 classes I. and II. But, since this separation has not yet been 

 actually effected, and since the possibility, that a unit {einheit- 

 lich) molecule may possess both kinds of fluorescence simulta- 

 neously, cannot be discarded h priori, Herr Lommel felt 

 compelled still to retain class III., i.e., bodies possessing com- 

 pound fluorescence. 



By artificially mixing substances of the first and second class, 

 fluorescences of the third order may be obtained in great variety. 

 In this way wonderful effects of colour are often obtained, and 

 they may be rendered still more astonishing by the addition of 

 strongly coloured non-fluorescent substances. 



Thus we see that yet other compound fluorescences than those 

 belonging to class III. are possible, and may, indeed, be pro- 

 duced artificially; viz., confining ourselves to only two fluores- 

 cent substances, by mixing two substances of the first or two 

 substances of the second class. A mixture of the first kind (of 

 naphthaline-red with fluoresceine, or with eosine, for instance) 

 may be easily recognised as such when examined by its spectrum ; 

 their fluorescent spectrum consists of two parts separated by a 

 minimum, neither of which follows Stokes's rule, but of which 

 the second refrangible part (belonging to the fluoresceine or 

 eos'ine) disappears, as soon as the incident homogeneous light has 

 been diminished down to that limit, where it ceases to cause 

 fluorescence in these substances. It is, however, more difficult 

 to recognise a mixtm'e of two substances of the second class as 

 such, as it behaves like a simple substance of that class. It 

 is possible that amongst the substances enumerated in class II. 

 there are mixtures of this nature, consisting of two or more 

 substances not separated hitherto. The varying behavioiu- of 

 the extracts of soot could, for instance, be easily explained by 

 tte supposition that in soot there are contained two or more 

 fluorescent substances of the second clas.', which are dissolved 

 in vaiying proportions by the dissolving media. 



Herr Lommel did not feel justified in establishing separate 

 classes for the two kinds of compound fluorescence just de- 



