534 



KNOWLEDGE & SCIENTIFIC NEWS. 



[September, 1906. 



posiiiblu, and an illustrated article by Mr. Arthur 

 J'ayne, published a few weeks ago in the " Hritish 

 Journal of I'hotography," gives some idea of the 

 necessary conditions. The plates used were bathed 

 in an " orthochronie T " solution, and so made ex- 

 tremely sensitive, especially to yellow light. The lens 

 had an ajjerlure of //.v'J (that is, live or six times as 

 large an aperture as an ordinary "rapid" lens), and 

 the exposure was one-lenth of a second. The subjects 

 were iigures on the stage, lit in the ordinary methods 

 during puljlic performances. The developmenl was 

 done at 75" i-ahr., and the negati\es were very thin. 

 As vigorous a transparency as possible was made in the 

 camera, using- a slow plate, and the process was re 

 peated on the transparency to get another negati\e. 

 15y then using a printing paper that gives great con- 

 trast, the final print was obtained. That is, with such 

 a rapid plate, fast lens, powerful light, and forced de- 

 velopment, the one-tenth of a second gave such under- 

 exposure that it required three consecutive struggles 

 after vigour to get a suitable print. 



Critidil Focussing. — The focussing screens generally 

 supplied with the best cameras are quite serviceable for 

 the more ordinary purposes, including the photography 

 of architectural subjects, but fail altogether when really 

 critical definition is required. They fail because they 

 are not flat, and also because the grain is not, as a rule, 

 line enough. A home-made screen will be found better, 

 and can be prepared without the exercise of much skill, 

 by getting two pieces of polished patent plate glass of 

 suitable thickness and the required size, and grinding 

 one side of each together until they present, when 

 washed and \siped, an evenly greyed surface. The 

 grinding is done by putting one piece of glass down on 

 a fiat board with small brads to prevent it slipping 

 about, but not projecting so high as the upper face ol 

 the glass, and working the other glass with a circular 

 motion on it, with fine emery and water between. Tlie 

 emery should be the finest that will cut, obtained by 

 elutriation. The finer the grain of the screen the less 

 bright is the image that falls on it, but the finer the 

 detail, therefore such a screen as described might not 

 be so good for portraiture and views as one with a 

 coarser grain. 



But for really critical focussing the surface of the 

 screen must be polished so that the image may not be 

 broken up at all. It is often stated that in such a case 

 a high-power eye-piece must be used, that the accom- 

 mfxlation of the eye may not introduce uncertainty. 

 This is an error, for no practical eye-piece could prevent 

 a normal eye from accommodating or focussing itself 

 so as to clearly see the image as it was moved over a 

 considerable distance. There must be marks on the 

 screen surface, and these marks must be seen in sharp 

 focus at the same time as the image; then the marks 

 and the image will be in the same plane irrespective 

 of the power of the eye-piece and the accommodating 

 power of the eye. A common suggestion for getting a 

 clear spot on a ground glass screen, is to cement a 

 microscope cover glass to it with Canada balsam. If 

 this is done a few lead pencil marks should be made 

 first to focus the eye-piece on. I have tried this method 

 a few times, but never succeeded. Of course, the glass 

 and balsam that the light passes through when focuss- 

 ing, but not when exposing the plate, will cause the 

 image plane to be different in the two cases, and 

 whether or not this is the reason, the fact remains that 

 I have not found this method reliable. If a clear or 

 nearly clear spot is wtuited on a ground glass screen, 

 the method that is the most successful with me is to 



rub the part w ith grease until the desired effect is pro- 

 duced. I ha\ e tried a great many oils and fats for this 

 purpose, and find that the simple cerate of the I'har- 

 macopteia is the best. 



If the whole screen is to be polished, then a selected 

 piece of patent plate is employed without grinding it, 

 and it remains to get the lines on its surface. It will 

 be found ad\antageous if these lines are ruled in groups 

 of three, with a greater .space between each group and 

 the next than between the members of the group. No 

 method of hand ruling with a diamond has been suc- 

 cessful with me, the lines arc broken and irregular, but 

 perfect results may be obtained by etching. For this 

 purpose, the selected piece of glass is thoroughly 

 cleaned and dried, then warmed so that when rubbed 

 with a piece of wax, the wax melts and coats the glass. 

 When cold, the film of wax should be complete and 

 thin. To make the lines, which, of course, must go 

 quite through the wax, a needle is not so good as a 

 sharp knife, and this should be drawn along against a 

 straight-edge, exactly as if cutting, but with a light 

 pressure, only just sufficient to go through the wax. 

 For the etching, a piece of thin sheet-lead is bent up at 

 its edges witfi the fingers, to form a shallow tray. 

 This should be placed out of doors on something level 

 and firm, enough strong sulphuric acid put in it to form 

 a shallow layer, and powdered fluorspar scattered evenly 

 on to it. Hydrofluoric acid is soon evolved, and the 

 glass, with its waxed side downwards is placed over it, 

 resting on the edges of the dish. In a short time, a 

 minute or two, perhaps, the etching will be sufficient. 

 The glass is warmed, the melted wax wiped off, the 

 glass well cleaned, and the lines examined with an eye- 

 piece. The best amount of etching should be deter- 

 mined by experiment, noting how long the glass re- 

 mains on the lead dish. Those not accustomed to 

 hydrofluoric acid need warning that they should be very 

 careful not to inhale any of it, or to allow the gas to 

 come in contact with their person in any way. But 

 with no more than ordinary care, and working out of 

 doors as described above, there is no reason to fear 

 trouble. The dish, after use, should be washed out 

 with a copious supply of water before bringing it into 

 the house. 



Wraf/cii's Panchromatic Plate. — Referring to mv re- 

 marks on this last month, the makers inform me that 

 it is a bathed plate. This therefore accounts for its 

 especial properties. 



The Committee 0! Bibliography, and of Astronomical Sciences, of 



the Koyal Observatory of Belgium, has undertaken to publish 

 a list of the observatories and astronomers of the whole world. 

 A request for information, in the form of a list of questions, 

 with a model reply relating to the Astronomical Service at the 

 Uccle Observatory, Belgiuiii, has been addressed to all the 

 directors of observatories. In addition the list will include 

 such astronomers (University professors, amateurs, cS:c.), who 

 are not attached to any observatory, but are nevertheless 

 actively engaged in astronomical research. The information 

 already sent will enable the Committee to draw up not only a 

 list of observatories, with their geographical co-ordinates and 

 the members of the staff, but also a table showing the astro- 

 nomical activity of the whole world, thanks to the facts given 

 as to the instruments at the disposal of each institution, the 

 pieces of research undertaken, and the papers published. We 

 are asked to appeal to the directors of those observatories 

 who have not received the question-form, or who have not 

 \ et forwarded a reply, as well as to unattached astronomers, 

 and trust they will send the information desired, or to repair 

 any omissions, as soon as possible, addressed to the Chair- 

 man of the Committee, Prof. V. Stroobant, astronomer at 

 the Royal Observatory of Belgium, Uccle, Belgium. 



