44S 



kxu\\li:dge. 



November. 1911. 



have a small piece of white card pinned to the edge of the 

 upright wood. This ser\es as a reflector and prevents the 

 shadows being too dark. 



The special points .about this easily made piece of apparatus 

 are that it calls for no alteration of the camera in use. though 

 one may have to buy an extra long camera screw ; and it 

 enables one to move the object and camera together, so as to 

 get any kind or direction of lighting. If a passing vehicle sets 

 up vibration in the room where work is going on the whole 

 apparatus is equally affected. _ 

 so that the definition docs not 

 (apparently) suffer. 



Figure 5 shows an ordinary 

 result obtained by this contriv- 

 ance. Figure 6 shows us a life- 

 size view of two cowrie shells 

 stuck on a card [vide Figure 4). 

 Figure 5 shows us a magnified 

 image, enlarged about 7 dia- 

 meters, of one of these shells. 

 It may be of interest to say at 

 once that the negative of Figure 5 

 was taken not with a lens, but 

 with a "pinhole" (/.c. made 

 with a needle) in a thin piece 

 of sheet copper. This sheet 

 metal, obtainable from any metal 

 dealer, is about the thickness of 

 a thin visiting card. A circular 

 piece is cut as an easy fit to the 

 inside of the lens tube. The 

 glass parts of the lens are un- 

 screwed and laid aside. Thi^ 

 disc of metal is pushed up insidt 

 the tube so as to rest against the 

 stop or diaphragm, and the centre 

 pierced with a No. 1 needle. 

 The hole which passes the fattest 

 part of this needle is about 

 .i'„-inch in diameter. In this case 

 tlie hole was about 14 inches 

 from the plate and about 2 inches 

 from the object. Although a 

 pinhole does not give the 



definition sharpness of a lens at its best for any one plane, yet 

 the pinhole give us a certain evenness of soft definition which 

 for some objects is particularly effective. In photographic 

 parlance, the pinhole gives us great depth but poor definition. 

 .\s a matter of fact this nV-inch hole is too large for best pinhole 

 definition at 14 inches. .\t this distance we slionld get a 

 sharper effect with about ;jV-inch hole. i.e.. a No. 5 needle. The 

 reader, will of course, understand that a good modern 2 -inch 

 lens, duly stopped down to say F.44 would have given a sharper 

 picture over a certain depth of subject, but would have 

 brought into prominence the differences of definition of the 

 various planes in an unpleasant way. This more or less hemi" 

 spherical subject was chosen as typifying a markedly difficult 

 case as regards depth of subject, and thus showing that at 

 times leg. fossils showing form rather than detail) the 

 homely pinhole gives a more effective rendering than the 

 costly lens. I emphasise this point because tlie use of a pin- 

 hole for this class of work is practicalK- unknown to most 

 workers. 



The following table shows one the best size of hole for 

 sharpest definition at various plate distances : — 



Plate Distance 6 8 10 I.t 20 JO inches 



Diameter of Hole ... s'„ A ;V .h. -S ■/.. •■ 

 Number of Needle... 9 8 7 .t 2 1 



The sizes of needles of various numbers vary slightly among 

 different makers ; but the above sizes, which are those of 

 Mil ward's Sharps, may be taken as generally representative. 



Of course, exposures with a pinhole are much longer than 

 with ordinary lens apertures ; but. beyond the matter of 

 patience, this is of no account with non-moving objects. 

 .Although it is wrong in theory, yet it works out all right in 

 practice to base exposure on the F. value of the pinhole. Thus 



Figure 5. 



a hole iV"" inch diameter, at 10 inches from the plate, may 

 be reckoned as F.400. 



To compare this with a lens working at F.32, for instance : 

 the squares of 52 and 400, viz., 1,024 and 160.000 are roughly 

 1 to IfiO. so that ten seconds with F.32 would be equivalent 

 to (about) twenty-seven minutes. But as a little extra 

 exposure is better th.ni under exposure one would in such a 

 case give thirty nuiiiitc;.. There is the further consideration 

 with pinhole expni-ures. \ i/.. that the occasional periods of 



\ ibration due to passing vehicles 

 are usually negligibly small in 

 comparison to the total time nf 

 exposure. 



F. C. Lambert. 



M.A., F.K.P.S. 



PHY.SICS. 



Hv Alfred C. G. Egerton, 

 B.Sc. 



ol'llTfl AI.MIA elec- 

 tric a. — L. Bell has published 

 recently in " The Proceedings of 

 the .American .Academy" an 

 investigation on glasses used as 

 a protection against the injurious 

 effect of light on the eye. 

 Bacteria are rapidly killed by 

 radiations of a smaller wave- 

 length than 270 A^M in the ultra- 

 violet, while the ultraviolet radia- 

 tions are absorbed by air con- 

 siderably if less than 230 /J-t^, and 

 by glass if less than 310 MM. The 

 symbol is an abbreviation for a 

 micro-millimetre or one-millionth 

 of a millimetre ; thus the wave- 

 lengths of the ultraviolet region 

 range from about three hundred 

 and sixty millionths to about one 

 hundred millionths of a millimetre 

 in length, shorter than which they 

 are so readily absorbed that 

 they have not yet been measured. All ordinary glasses absorb 

 the ultraviolet rays which have cell destroying power or 

 bactericidal effects. Common amber-coloured glass cuts oft' 

 all the ultraviolet region. In contra-distinction to this a yellow 

 filter of gelatine soaked in dimethylaniline permits some of the 

 longer ultraviolet waves to pass, but cuts off a large portion of 

 the visible region, and can, in conjunction with other screens, 

 cutting oft" the yellow {e.g.. cobalt glass), be used to filter 

 through the ultraviolet rays. 



.Arc lamp workers, and those that work electric furnaces. 

 which, by the way. are in use now for preparing high grade 

 steel from rich Norwegian ores, are rarely affected by true 

 " ophthalmia electrica " chargeable to the ultraviolet rays, but 

 are very often affected by the intense luminous radiation. The 

 vitreous humour and crystalline lens of the eye is capable of 

 absorbing most of the ultraviolet radiation, but there appears 

 to be some particular wave-length which gets through rather 

 easier, and is apt to cause trouble. The remedy is to wear 

 glasses when dealing with arc lamps or quartz mercury vapour 

 lamps; while, to prevent the injurious effect of the hnninous 

 radiations, glasses should be slightly tinted. 



It is interesting to compare the transparency of quartz, 

 uviol glass and an ordinary photographic lens to ultraviolet 

 waves. The photographic lens does not let through waves 

 shorter than 365 mm. while uviol glass, a glass made in the 

 Schott and Genossen works at Jena, allows through light of 

 265 /J-p- and quartz even shorter waves. 



SliLENICM. — This element exists in se\eral modifications 

 called in the language of the chemist, allotropic forms. 

 There is a red amorphous form, a grey crystalline form and a 

 darker, almost black, form. .Ml these three forms, though 



Figure 6. 



