154 



KNOWLEDGE. 



May, 1915. 



described an objective thai bad been made for him by Messrs. 

 Beck in 1909. It is a half-inch dry lens, fitted with a front 

 Carrying an oil-immersion lens. It was described in the 

 catalogue oi the Optical Convention in 1912. Its properties 

 and advantages were explained. They resembled those 

 obtained with high-power immersion lenses, and which 

 had previously been considered to be obtainable Only with 

 powers of one-eighth inch or one-twelfth inch. 



\ paper In Mr. Harris on "Microscopical Methods in 

 Brvological Work " was read by Mr. F. j. Perks. It dis- 

 cussed various media suitable for mounting mosses, with 

 the methods that should be employed in order to display 

 the structure as well as the whole plant. 



For tickets of admission to the meetings, application 

 should be made to the Honorary Secretary, Mr. James 

 Burton, S, Somali Road, West Hampstead, N.W., or to any 

 of the leading London opticians. 



J B. 



PHOTOGRAPHY. 



By Edgar Senior. 



MAKING NEGATIVES READY FOR PRINTING— 

 Apart from retouching, a great deal can be done to improve 

 the printing qualities of a negative by the expenditure of 

 a little time upon each one. After drying. — which operation 

 should be carried out as far as possible in an even tempera- 

 ture, and should not occupy more than three or four hours — 

 the negative should be examined carefully for any defects, 

 such as pinholes, air bells, scratches, and so on, and these 

 should be carefully touched out by means of a fine sable 

 brush and water colour. Almost any pigment which will 

 not allow the chemical rays to be transmitted will answer 

 the purpose ; thus Indian red or carmine have both been 

 largely employed, as well as a specially prepared substance 

 known as " Gihon's opaque." A common mistake with 

 beginners when spotting is that they use the colour too 

 thin, when, in the case of a transparent spot, the colour 

 runs round the hole, and leaves the centre bare, and this has 

 afterwards to be filled in. The trouble may also arise from 

 the use of too large a brush. The colour must not only be 

 used as dry as possible, but should be applied with a very 

 small sable brush, such as a § of Newman, or a series 15 of 

 Winsor & Newton — camel-hair is useless. Pinholes can, 

 as a rule, be spotted out by a single touch, whereas large 

 spots, such as those due to air-bells, require to be filled in 

 by means of a succession of small touches, applied in such 

 a manner as to bring the density up as nearly as possible 

 to that of the surrounding parts of the image. Scratches 

 on the film, if only surface-formed, can very often be obli- 

 terated by means of a finely pointed pencil. When the sky 

 in the negative of a landscape or architectural subject 

 is defective, it is often better to block it out completely. 

 This may be accomplished as follows. Make a rough print 

 from the negative on P.O. P. paper, and cut out the sky 

 portion at a distance of about one-sixteenth of an inch 

 above the outline of the landscape portion. The sky of 

 print is then exposed to daylight to blacken, and while this 

 is taking place the outline of the sky on the negative is 

 blocked out for about a quarter of an in / "h all along. The 

 sky portion of the print having become blackened is now- 

 used as a mask by attaching it to the negative — on the film 

 side — by means of a few touches of gum. The slight amount 

 of trouble taken in painting out round the outline of the 

 landscape is more than compensated for in allowing the 

 sky mask to be more or less roughly cut out ; and, besides, 

 no matter how accurately this may have been cut, it is not 

 easy to fit it to the negative when the outline cf the subject 

 is at all intricate. In the case c f architectural or other 

 subjects in which there may be a series of straight lines, 

 it is often found very difficult to outline these with a brush 

 without producing the appearance of a broken or wavy line. 

 A better plan would be to use a drawing-pen and a straight- 

 edge, and charging the pen with Indian ink, to hold it ver- 



tical or at right angles to the negative, and without bearing 

 too heavily upon it, proceed to draw a good thick line along 

 those parts of the building having straight lines standing 

 out against the sky, and then, afterwards, with the brush 

 proceed to fill in any little corners where the drawing pen 

 could not be used. Having outlined the whole, the entire 

 surface of the sky may now be gone over with the brush, 

 bringing it just to touch the lines made by the drawing-pen, 

 by this means completing the blocking-out of the sky or 

 background. 



EMPLOYING USED FERROUS OXALATE DE- 

 VELOPER. — It may be of interest to some to know that 

 ferrous oxalate developer that has been used, instead of 

 being thrown away, may be employed to provide potassio- 

 ferric oxalate, this substance being the basis of one of the 

 best reducers, which is known as " Belitski's reducer." 

 All that is necessary is to place the used developer in a dish, 

 so that it is freely exposed to the air, when the emerald- 

 green crystals that are deposited are collected and rinsed in 

 cold water, to free them from any oxide of iron which may 

 adhere to them. This being done, the crystals are dried on 

 filter paper, and placed in a well-stoppered bottle, and kept 

 in the dark, as potassio-ferric oxalate is somewhat sensitive 

 to light. In order to prepare the reducing solution, thirty 

 grains of the salt are dissolved in half an ounce of water, and 

 then mixed with half an ounce of the fixing bath containing 

 four ounces of " hypo " dissolved in twenty ounces of water. 

 The negative which requires reducing is placed in the 

 solution contained in a dish, which is kept rocked during the 

 operation. Reduction soon commences, and proceeds 

 regularly, and when finished is followed by a thorough 

 washing under a stream of water, which completes the 

 operation. When done with, the solution is thrown away, 

 as the mixed solutions do not keep, 



PHYSICS. 



By J. H. Vincent, M.A., D.Sc, A.R.C.Sc. 



DETERMINATION OF TEMPERATURE BY 

 MEASURING RADIATION.— The temperature of a 

 source of radiant heat, such as the interior of a furnace, 

 may be deduced from observations on the radiations which 

 it gives out. If one assumes that the interior of the furnace 

 behaves as an ideal black body, as will be the case if the 

 observation is carried out through a small break in the 

 enclosure to the furnace, then by Stefan's law the total 

 radiation is proportional to the fourth power of the absolute 

 temperature. The radiation can be concentrated on to 

 the surface of some form of bolometer, or on to a thermopile, 

 by means of a mirror or lens ; and the instrument can be 

 calibrated by the use of sources of known temperature. 

 The Fery radiation pyrometer is an instrument of this class. 

 It consists of a reflecting telescope, which is pointed at the 

 interior of the furnace through a hole in the wall. After 

 reflection from the face of the concave mirror the radiation 

 falls on a thermo-couple placed at the conjugate focus 

 to the hole in the furnace wall, and thus situated between 

 the face of the mirror and the hole. On the other side of 

 the mirror is an eyepiece by means of which the observer 

 looks through an aperture in the centre of the mirror, and 

 adjusts the image of the opening in the furnace until it 

 slightly overlaps the disc which forms the absorbing surface 

 of the thermo-couple. A suitable galvanometer is connected 

 in series with the thermo-couple, and its deflections are 

 proportional to the fourth power of the absolute temper- 

 ature of the source. 



OPTICAL PYROMETERS.— These instruments, of which 

 several forms are in use, all have a photometric basis. 

 They compare the intensity of the red rays from the source 

 with the intensity of the red rays given by a standard lamp. 

 For example, in the Holborn-Kurlbaum pyrometer the 

 filament of an electric glow-lamp is placed in the focus of 

 the telescope which views the source. The current through 



