May, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



113 



Photography. 



Pure arvd Applied. 



By Chapman Jones, F.I.C, F.C.S., &c. 



Photographic Photometry. — It may seem to be a very 

 obvious suggestion and a very simple matter, seeing 

 that an increase of brightness of the light that impinges 

 upon a photographic plate, causes an increase in the 

 darkness of the resulting deposit, to use a photographic 

 method for the purpose of comparing the brightness or 

 luminosity, especially of those sources of light that cannot 

 be brought into the laboratory, such, for example, as the 

 heavenly bodies. The fact that the increase in darkness 

 of the deposit is not simply proportional to the increase 

 in brightness of the light (with equal exposure time) is 

 easily o\ercome by impressing a light-scale on the plate 

 so that this shall be subjected to whatever treatment, as 

 in development, &c., that the other exposed parts receive. 

 A light-scale is nothing more that a series of small 

 patches that have been exposed to a uniform light for 

 different times, generally so that the amount of light 

 impinging on the respective patches is proportional to 

 the simple series, 1, 2, 4, 8, 16, &c. By measuring the 

 opacities of these patches and plotting them against the 

 amounts of light,-a curve can be drawn that will show 

 at once the relationship between opacity and light in that 

 particular case. Since writing this, a suggestion has 

 actually been published to determine luminosity by esti- 

 mating by chemical means the metallic silver produced 

 on a photographic plate by the agency of the lights that 

 it is desired to compare. 



Thus far all is easy, and it is at this stage that the 

 thoughtless worker too often leaves off, considering that 

 he has completed his task. It is easy to teach students 

 to measure and to weigh, nothing of its sort is more 

 easy, but the difficulty begins with the consideration of 

 what it is that has been measured or weighed. The 

 student of chemistry weighs things and gives them 

 names, but the things he weighs rarely are what he calls 

 them, and in some cases I have known the material 

 weighed not to contain a vestige of the substance that it 

 was supposed to consist of. And so it is in other work, 

 the whole difficulty is to know what it is that has been 

 dealt with. In investigational work, there is a strong 

 temptation to move along the line of least resistance, and 

 this distinctly is to perfect the methods of measurement. 

 It is only necessary to get a little knowledge and a good 

 instrument maker, to reduce the differences between the 

 results of the repeated measurements of the same thing. 

 But if the thing measured is not what we take it to be, 

 if there is, for example, 10 per cent, of uncertainty here, 

 it is mere deception and waste of time to seek to reduce 

 the I per cent, of uncertainty in the method of measure- 

 ment. I am convinced that in a vast number of cases 

 of very many kinds instrumental perfection is already 

 far beyond what we can do justice to, and that the 

 pressing difficulties are the avoidance of loss, and the 

 more perfect isolation and more truthful recognition of 

 the thing that is measured. 



In the example quoted above in response to the sugges- 

 tion to use a photographic method for comparing light 

 intensities, it is not the brightness of the lights that 

 would be compared, nor is it their activism, nor their 

 radiant energy. It would be nothing whatever more 

 than a certain effect that they could produce upon a 

 certain sensitive surface. If the sensitive surface w-ere 

 varied the results would be different. The old idea that 



activism could be equally well measured by any chemical 

 change that light can produce, and that the selection of 

 the sensitive substance is a mere matter of convenience, 

 cannot, of course, be maintained, and so far as it remains 

 of use is a testimony to the clumsiness and the want of 

 discrimination of the methods that we employ. 



Brightness is essentially a matter of sight, and the eye 

 is therefore the only standard instrument for its measure- 

 ment. By putting over a sensitive plate a coloured 

 medium so exactly prepared that a continuous spectrum 

 photographed through it on the plate would give a density 

 of deposit truly proportional to the brightness of the 

 spectrum in all its various parts, a combination would 

 be obtained that would give proportional brightnesses if 

 used in the manner already described. 



Time Development as Afeded by Temperature. — In the 

 March number I made some remarks on the mechanical 

 method of development employed by some, in which the 

 exposed plates are allowed to remain in the developer for 

 a fixed and predetermined time instead of allowing one's 

 judgment to decide when the image is sufficiently dense. 

 I there pointed out what I believe to be the advantages of 

 such '• time development." In the March number of the 

 Journal of the Royal Photographic Society is published 

 a paper by Messrs. Ferguson and Howard, in which they 

 suggest that plate makers should give with their plates 

 the times necessary for development at various tempera- 

 tures with the formula they recommend. For a given 

 pyro. soda formula, which, by the way, has too little 

 sodium sulphite to secure a deposit free from the oxidised 

 products of the pyro., they find that "kodoids " give the 

 same steepness of gradation at 17" C in 6 minutes, as at 

 at I7p C in 7 minutes 25 seconds, or at 7'^' C in g minutes 

 50 seconds. They describe in detail a method of deter- 

 mining the relationship between time and temperature 

 when the contrast (or " development factor ") remains 

 constant. 



If makers of plates do this, and photographers, one 

 and all, do as the makers tell them, then photography 

 as an art may gain something in the ways I indicated 

 two months ago, but it will lose an incalculable range of 

 possibilities in the hands of the skilful. It is one thing 

 to use mechanical methods when the balance of advan- 

 tage appears to be in favour of them, but quite another 

 to seek to supplant all discretion by rigid rules. The case 

 may be compared to the feeding of convicts who have 

 their food w.eighed out to them, and the work expected of 

 them definitely catalogued. There is much advantage 

 in this exact balancing of work and food ; gluttony and 

 starvation are avoided and economy is secured. But we 

 who are not convicts do not weigh our food nor measure 

 our work, and think that on the whole we have reason 

 for believing that our health is rather better for the dis- 

 cretion that we prefer to exercise. We consider that our 

 experience is worth something. 



Iron Lightning Corvdvictors. 



Sir Oliver Lodge is reported to have expressed his 

 opinion in favour of iron lightning conductors in pre- 

 ference to copper ones. The former allow the current 

 to flow more gradually and to leak more slowly, while 

 with copper, especially if it be of large diameter, a more 

 sudden effect is produced, which may cause side flashes 

 and do damage. The iron rod may be fused, but only 

 after it has done its work, and it is easily renewed. A 

 lightning conductor should be looked upon as a safety 

 fuse, to be replaced when it has been struck. 



