May, 1911. 



KNOWLEDGE. 



195 



The instrument, in use. appears to be very convenient, the 

 greatest difficulty in photoi^raphic use, however, being due to 

 the colour effect of daylight, which makes matching distinctly 

 difficult. It is possible that this might be completely avoided 

 by the aid of an orange screen to tint the daylight beam, 

 but such a screen would require careful adjustment. 



There are numerous directions in which such an instrument 

 can be of use in photography ; in the first place, it is obviously 

 of importance to know the range of contrast which exists in 

 photographic subjects, and this is a point on which hitherto 

 experiments ha\e been difficult to make, and on which \iews 

 of considerable diversity have been expressed. 



Messrs. Hurter and Driffield measured the maximum 



t 



Figure 1. 



difference of intensity in landscape subjects by taking a 

 photograph of a scene in which was included a square of 

 white cardboard in sunlight and a piece of black velvet in 

 shadow ; the plate was developed together with another plate 

 from the same box which had been exposed behind a sector 

 wheel to a candle, and the densities of the patches representing 

 the white cardboard and the black velvet were measured and 

 compared with the densities of the known exposures on the 

 other plate. As a result the white cardboard in sunlight was 

 found to reflect thirty times as much light as the black velvet 

 in shadow. 



In their paper, Messrs. Dow and Mackinney mentioned that 

 the maximum range of intensity which they had measured in 

 a subject between the brightest white magnesia and the 

 darkest dead black which they had been able to obtain w-as as 

 thirty to one : so that apparently the result obtained by Messrs. 

 Hurter and Driffield for the maximum range in ordinary 

 photographic work may be considered as confirmed. 



Some measurements made by means of a similar, but much 

 less convenient, portable photometer two or three years ago. 

 gave the writer only one to four as the range of intensity in a 

 street scene on a dull day. so that probably the range in most 

 landscape photographs is of the order of one to eight or 

 one to sixteen, a range which is capable of being represented 

 within the " period of correct exposure " of a plate. 



.\nother use for the instrument, suggested by the authors 



of the paper, was the finding o» exposures, and although 

 several speakers in the subsequent discussion considered that 

 the variation in the actinic value of daylight would make the 

 instrument of but little value in this direction, it seems to me 

 that in some branches of photography it should prove of very 

 great value indeed. 



While it is im- 

 probable that such an 

 apparatus, which is 

 necessarily of some 

 size, will displace the 

 handy and simple 

 sensitive paper actino- 

 meter for landscape 

 work, yet in the 

 technical studio it has 

 many advantages. In 

 copying prints by day- 

 light in this climate 

 difficulty is often in- 

 troduced by the rapid 

 variation of the light 

 while working ; even 

 if the first exposure 

 pro\ e quite correct on 

 development, the light 

 may change sufficiently 

 to spoil it before a 

 second exposure is 

 made. 



The use of an 

 actinometer in such 

 work is almost im- 

 possible because of 

 the time required for 

 the exposure of the 

 actinometer paper, but 

 the ■■ Lumeter,"' which 

 enables a measure- 

 ment to be taken in a 

 few seconds, would 

 probably save man\- 

 plates. In black and 

 white work, again, 

 where what is required 

 is the maximum ex- 

 posure which can be 

 given without pro- 

 ducing any deposit in 

 the parts of the plate 



representing the blacks in the original [all that jwould rbe 

 required to render the calculation of the exposure absolutely 

 mechanical would be to find out what exposure to one foot 

 candle for a given plate and stop would just fail to produce 

 an impression upon the plate, and then make a measurement 

 of the light reflected from the blacks of each subject. 



In enlarging, also, a measurement of the light falling upon 

 the enlarging easel through the densest parts of the negative 

 should, after one or two preliminary trials, enable the correct 

 exposure to be calculated with considerable ease. 



A HYDROSTATIC APPARATUS ILLUSTRATING 

 THE LAW OF DEVELOPMENT.— The development of a 

 photographic plate proceeds according to the equation which 

 represents what in chemistry is called a mono-molecular 

 reaction of the first order, although development is. of course, 

 a heterogeneous reaction, of which the surface of the silver 

 bromide is the \ariable. The law of de\elopment is 

 very simple, and is, that the rate of development at an\- time 

 is equal to a constant to the difterence between the ultimate 

 density that can be obtained if the development be indefinitely 

 prolonged and that which already exists, that is: Rate = 

 Constant iDco— D). 



In order to make this plain a little piece of apparatus can 

 be made which supplies an almost perfect analogy. It consists 



Figure 2. 



