plate. Here the results of my experiments differ from those of Tschirch, who found but one band in the zinc and copper compounds, but the discrepancy may have arisen from the fact that he does not state in his memoir what acid was in combination along with the metal with phyllocyanin. That the zinc carbonate compound of phyllocyanin should give two bands in the violet region corresponding very nearly in position and relative intensity with the two of phylloxanthin, and therefore, with the second and third violet chlorophyll bands, is interesting from the fact that of all the chlorophyll derivatives the spectrum of the phyllocyanin zinc carbonate in the visible region corresponds more closely than any of the others with that of chlorophyll, and from the deductions arrived at from this similarity by Schunck* as regards the functions of chlorophyll as being a carrier of carbonic acid in the plant, just as hæmoglobin serves to convey oxygen in the animal economy.

Alkachlorophyll, Phyllotaonin, Ethyl-phyllotaonin, Phylloporphyrin.

In his "Contributions to the Chemistry of Chlorophyll" Schunck has discovered that by the action of alkalis upon chlorophyll, the above definite derivatives are produced in a crystalline form.t Alkachlorophyll, phyllotaonin, and ethyl-phyllotaonin all give the single band in a pronounced and well-defined manner, corresponding very nearly in position with the band in phyllocyanin, viz., at about Ks, in phyllotaonin it being shifted slightly towards the violet, and in the other two a trifle towards the red end of the spectrum (Plate 5, figs. 6, 7, and 8). Alkachlorophyll also forms a definite crystallised sodium salt. In an alcoholic solution the

band appears considerably shifted towards the red, but a watery solution gives the band much obscured, in the same position as the one in the alcoholic solution of alkachlorophyll itself (Plate 4, figs. 9, 10, and 11).

Phylloporphyrin on the other hand gives a double band, intense and fairly well defined, the less refrangible one having its centre situated at the Ks line (Plate 5, fig. 9). Phylloporphyrin forms a compound with acids, which has a spectrum in the visible region quite distinct from the very characteristic one of phylloporphyrin itself, which consists of seven bands, while the acid compounds only show three bands. On examining the hydrochloric acid compound in the violet region, a corresponding change is also found in the spectrum, and instead of the double band, a single very intense * 'Annals of Botany,' vol. 3, pp. 65—120.

Roy. Soc. Proc.,' vol. 39, p. 355; vol. 44, pp. 449–454; vol. 50, pp. 312-316; vol. 57, pp. 316-319.

Schunck and Marchlewski, Roy. Soc. Proc.,' vol. 57, p. 319.

one takes its place, situated between the other two (Plate 5, fig. 10). This band is the most pronounced and the best defined one in the whole series, and only becomes visible on the photographic plate in excessively dilute solutions, so dilute that one might say the solution was colourless to the eye when viewed by transmitted light.

Phylloporphyrin and Hæmatoporphyrin.

On comparing the spectra of phylloporphyrin and hæmatoporphyrin in this region, and also those of their hydrochloric acid compounds (Plate 3, figs. 6, 7, 8, and 9), it was found that hæmatoporphyrin gave only a single band, but situated in the same position as the double one of phylloporphyrin. On this point the results of my experiments differ from those of Tschirch, who, as stated above, found in both a single band occupying the same position. In the hydrochloric compounds of hæmatoporphyrin, however, a single band of the same pronounced character as that in phylloporphyrin was found, the one in hæmatoporphyrin, as will be seen from the figures, being slightly shifted towards the red end of the spectrum, which is interesting from the fact that just in the same way are the bands in the visible region of the spectrum of these two compounds shifted, this constituting their only spectroscopic difference.

In conclusion, my thanks are due to Dr. E. Schunck and Dr. L. Marchlewski for the valuable assistance they have given me in many details in connection with this investigation.

I hope in a further paper to investigate more particularly the spectroscopic behaviour in the same region of the spectrum of the yellow colouring matter accompanying chlorophyll in leaves and allied colouring matters obtainable from other sources than the leaf, for instance, carotin.

"On Photographic Evidence of the Objective Reality of Combination Tones." By R. W. FORSYTH, A.R.C.S., and R. J. SOWTER, A.R.C.S. Communicated by Professor RÜCKER, Sec.R.S. Received March 29,-Read May 5, 1898.

[PLATES 6, 7.]

In the following paper we propose to describe a series of photographs which prove the objective reality of difference and summation tones. The work was suggested to us by Professor Rücker, and we have used the method of detecting these tones which has been described by Rücker and Edser in the Philosophical Magazine' for April, 1895.