March, 1907 ] 



KNOWLEDGE & SCIENTIFIC NEWS. 



63 



Photography : Pure and Applied. 



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



Attention has recently been emphatic- 

 Variations ally drawn to the fact, well enough 



in Exposure known to many of us, that a large 

 Shutters. proportion of the exposure shutters 



now available, not only do not give 

 exposures of the duration marked on them, but 

 that they too often give variable exposures under what 

 are supposed to be the same conditions. A shutter 

 may be adjusted with the utmost precision by the 

 maker, but if there is much friction in its various parts, 

 a little change in the nature of the surfaces of those 

 parts, or a little dust or grit, will make a very con- 

 siderable difference in the duration of the exposure 

 and render the maker's adjustment useless. I do not 

 refer only to those shutters that can never possibly give 

 the exposures marked upon them, but also to those that 

 are honestly made and honestly marked. 



It is obvious that this variation, due to alterations in 

 the rubbing surfaces and to the presence of extraneous 

 matter upon them, is likely to increase as the surfaces 

 are more changeable and more extensive. Other circum- 

 stances being equal, a focal plane shutter with its large 

 blind is more likely to suffer than a lens shutter with 

 its smaller blind, and this, again, than a diaphragm 

 shutter with its small rubbing surfaces of smooth metal. 

 On the other hand, a little grit, that would be fatal to 

 the performance of a metal shutter with its unyielding 

 surfaces, might be carried along on a sheet of fabric 

 without making any sensible difference to its rate of 

 movement. The best protection that is possible for 

 such apparatus cannot keep dust out nor prevent 

 material, whether fabric or metal, from being affi 1 ted 

 by friction, and presumably the softer substance will 

 suffer most. 



To minimise irregularities it follows that the moving 

 parts should be small, that they should be hard (that is, 

 of metal), and that the rubbing surfaces must be re- 

 duced to the very smallest dimensions. Twenty-two 

 years ago I published a description of a shutter that 

 consists of a disc supported on a central pin. with a 

 sector-shaped in it that, when the disc revolves in 

 its own plane in front of the lens, gives tin- required 

 exposure. The opening in the disc is adjustable b) 

 a sector-shaped shutter to vary the exposure. Shortly 

 after, I had made by a watchmaker a shutter on exactly 

 the same lines, but the disc was done away with except 

 a sufficient part of it to cover the lens. There were 

 really two sectors made of very thin brass that could 

 be put one over the other for the maximum exposure or 

 fanned out for a shorter exposure. I believe no shutter 

 could give more constant results than this, for the only 

 friction is on the small central pin. and 1 cannot 

 ceive it possible to further reduce the friction. The 

 rotating part can be removed in a moment and 

 cleaned if necessary, but in this country it never needs 

 it. That is the shutter that I should take to countries 

 where dust and sand are so troublesome. It worked 

 perfectly lor many years, and is still as good as new, 

 for it has nothing to wear out. The drawback to it is 

 the limited range of exposure that any such devil 

 give, unless, of course, the opening is reduced to less 

 than the lens aperture, when what some call the " e\ 



posure " would he reduced, though the time necessar} 

 lor the "exposure" to he given would remain much 

 about the same. With a large disc, or its equivalent 

 in se.tors, an extensive range is possible, bul lor port- 



The Photometry 



of 

 Coloured Lights. 



able apparatus it must be small, and, therefore, its 

 range must be undesirably limited. 



The paper on this subject, by Mr. 

 J. S. Dow, that was communicated 

 to the Physical Society of London last 

 May, has just been published in the 

 Proceedings of that Society. He 

 shows what grave differences are obtained when com- 

 paring a red and green light by merely altering the 

 size of the illuminated surfaces in a Joly photometer, as 

 well as by the use of other types of instruments. It is 

 well know n that the sensitiveness of the retina to differ- 

 ent colours is not equally proportioned in its various 

 parts, but the value of Mr. Dow's paper consists in the 

 fact that he has estimated the discrepancies that this 

 leads to in actual work with the current forms of instru- 

 ments. The subject is of fundamental importance to 

 all who are interested in the estimation of the com- 

 parative luminosities of coloured surfaces, as well as 

 to those who deal with what are more commonly under- 

 stood as coloured lights, and even the lights from 

 various sources that are generally regan lour- 



less or white are sufficiently different in their tin 

 give measurable and sometimes notable variations. 



The appearance of things in general to 

 Fishes'-eye animals that inhabit water has an in- 

 Views. terest other than that which concerns 



those who find enjoyment in catching 

 them. We know how rarely it is that water is still 

 enough and clear enough to allow us to see objects much 

 below its surface. In looking from under the water sur- 

 face the objects in the second medium (the air) are gener- 

 allv better illuminated than those in the water, and this 

 must be a considerable advantage, but the ripples inter- 

 fere greatlv with the definition of objects outside, 

 causing such confusion and apparent movement of them 

 that they may not be distinguishable. A bright or 

 well polished object would generally be more conspicu- 

 ous if on shore than in the water when seen from the 

 other medium, because in the former case it would be 

 more brilliantly illuminated. In the February number 

 of The Photographic Monthly there is a short and inter- 

 esting article on this subject by Mr. J. Allan Stewart, 

 M.A., illustrated by two photographs actually taken 

 from under water, using a pin-hole instead of a lens to 

 avoid the difficulties of focussing, etc. This is a curi- 

 ous subject well worth further Study, and Mr. Stewart's 

 is a valuable contribution to it, perhaps chiefly because 

 he shows a suitable and simple experimental method 

 for the purpose. 



The British Journal of Phot 

 has begun this year to issue an eight- 

 page supplement devoted to " Colour 

 Photography " with the first number 

 of the journal for each month. One valuable featUI 

 it is an illustrated summary in chronological ordi 

 British patents granted for this and allied Subj 

 beginning with Louis Ducos du Hauron's specification 

 of three-colour photography, dated 1876. I hope that 



in due time foreign patents will he dealt with as well. 



There has recently been established " The S 

 Colour Photographers," of which Mr. Henry J. 1 

 lev, of Surrey House, Stroud, Glos., is the Honorary 



Secretary and Treasurer, for the purpose of the inter- 

 change of experiences and specimens of work. (' 

 tions of specimens are already being circulated among 

 the members. I here must he .1 at number of 



persons interested in the Copying ol colour Ihv.iusc 



the applications of such work ate innumerable, and il 



matter foi heart] congratulation that they can now claim a 

 J and a journal devoted exclusive!) to t! 



Colour 

 Photography. 



