59° 



NATURE 



[October i8, 1S94 



well-ligbted studio (expiessed in normal candles) is 

 50,000-100,000 ; direct sunlight (October), 450,000 ; 

 electric arc light of Sooo candle visual intensity, 

 100,000-300,000 ; flash light with four grammes mag- 

 nesium powder, potassium chlorate and perchlorate, 

 10,000,000.' 



The chapter on sensitometers is particularly good, and 

 the critical discussion of the various systems in vogue 

 will be found most useful. It is true that the author 

 urges the advantages of his own " tubular photometer," 

 but this does not prevent his doing justice to other forms. 

 The discussion of the so-called law of photographic 

 reciprocity according to Miethe and .Michalke (chapters 

 viii. and i.\.) comes most opportunely at the present time. 

 The relationship between the density of the silver deposit 

 formed on development and the time of exposure, in- 

 tensity and quality of light, &c.. is a subject which has 

 given rise to a great amount of discussion in this country 

 lately. Strange to say, however. Dr. Vogel gives no re- 

 ference to the work of Abney, Hurter and Driffield, 

 and others who have taken part in the recent discussions. 

 This is certainly an omission. It appears that Dr. Miethe 

 undertook to investigate the accuracy of the law of 

 reciprocity in order to ascertain whether photography 

 could be applied to the measurement of the brightness 

 of the stars. [.-1 priori, one would have thought that the 

 ordinary photographic plate would be inapplicable to 

 this purpose because the photographic efficiency is de- 

 pendent on the quality of the light emitted by a star, 

 and this does not necessarily coincide with visual inten- 

 sity.] Among other points brought out by his investiga- 

 tion is the fact that for very feeble illumination the law 

 in question does not hold good, but when the light is 

 about four times the intensity necessary to produce a 

 visible result (on development) the relationship is true up 

 to looo times this intensity. Other deviations from the 

 law are discussed by Michalke in the paper which Dr. 

 Vogel partly reprints, but enough has been said to indi- 

 cate the importance of these two chapters. 



The portions of the work to which those who have 

 followed the recent developments of photography will 

 naturally turn with great interest, are those dealing with 

 the action of special sensitisers, a subject which will 

 always be identified with Ur. Vogel as the discoverer to 

 whom we are indebted for this advancement. The 

 greater portion of the volume — viz. chapters xvi. to 

 xxvii. inclusive — are devoted to this and related subjects. 

 In connection with the sensitiveness of the silver haloids 

 to the colours of the spectrum, the author brings out one 

 point very clearly, and that is the futility of using the 

 sun as a source of light in such experiments. It appears 

 that the photographic transparency of the atmosphere 

 is subject to such very great lluctuations, that the maxima 

 of chemical activity are apt to be considerably shifted 

 from day to day, so that concordant spectrum prints 

 must not be expected from the solar spectrum in cases 

 where accuracy is required. The action of organic 

 colouring matters as special sensitisers receives very full 

 treatment, and all the recent investigations on this sub- 

 ject are brought together. Of these the experiments of 

 Dr. E. Vogel on the colour sensitiveness of the eosin 



* 'I'tlCfC liumljers arc referred lo the /<hi)tngra/>hii, nol visual intensity of 

 the ttandard canalc. 



power increases as 



■SO. I 303, VOL. 50] 



colouring matters by themselves (chap, xx.), and on the 

 special sensitising action of the pure colouring matters 

 of this group on gelatino-bromide emulsion films 

 (chap, xxii.), are of considerable importance. It appears 

 now that the fugitive character of these colouring matters 

 is due to photochemical reduction, and not oxidation 

 (pp. 163-164) ; but the evidence on this point does not 

 appear to the writer to be quite conclusive. It is shown, 

 further, that the best of these colouring matters as special 

 sensitisers are those which are by themselves the most 

 sensitive to light— viz. tetraiodo- and diiodo-tluorescein— 

 that the silver salts are better than the alkaline salts for 

 this purpose, and that the sensitisin; 

 the fluorescent power diminishes. 



Other organic colouring matters are treated of in con- 

 nection with this same photographic property, and the 

 details, as given in the book, will be found well worthy 

 of study. We may here call attention to the interesting 

 work of Wollheim (chap, xxiii.) on chlorophyll, from 

 which it appears that the efticient special sensitiser in 

 the case of this substance is the phyllocyanin. The un- 

 certain action of chlorophyll is well accounted for by 

 these researches. Among recent work on the action of 

 organic colouring matters, the author gives an account, 

 almost verbatim, of that published by Mr. J. Acworth in 

 1S90. This writer has made a detailed study of all the 

 most efficient special sensitisers, and, as a general con- 

 clusion, confirms the view that the maxima of absorption 

 and chemical activity do not absolutely coincide, but 

 that a displacement occurs in accordance with a law 

 which has been developed theoretically by E. Wiede- 

 mann. Figures of some of the absorption and photo- 

 graphed spectra are given (p. 192), and a plate giving 

 Acworth's curves for the various colouring matters. 



We might have dwelt upon many other topics discussed 

 in the book ; enough has been said, however, to show that | 

 photographic literature has been enriched by a work 

 which will take rank among the classics in this subject. 

 If, in concluding this notice, some defects are pointed 

 out, it is not that the writer feels bound as a critic to find 

 fault with something, but because in a work of high 

 standard, such as this, small faults obtrude themselves 

 and become great by contrast. In the first place, then, 

 the author does not give sulticient recognition to work 

 done in this country. One such omission has already 

 been pointed out. Similarly (he work of Abney on 

 colour measurement, which is so closely related to the 

 subject-matter of chapter xxx., is not referred to. Although 

 J. J. Acworth's experiments on orthochromatic photo- 

 graphy are given in exUnso (because originally published 

 in M'icilcntanns Annalen .'), the prior work of C. H. 

 Bothamley is not described. In connection with the 

 theory of direct photography in colours by the method of 

 interference, Zencker alone is mentioned, and all refer- 

 ence to Lord Kayleigh's work on this subject omitted. 

 But still more serious is the introduction of polemical 

 matter into the work— in some parts to such an extent as 

 to become an actual disfigurement. The whole of 

 chapter xvii., on the history of the discovery of the 

 action of special sensitisers, might very well have been 

 omitted ; what is the use of reprinting a series of 

 polemical papers published twenty years ago, especially 

 when one of the combatants (SchuUz-Sellack) is dead 



