PHUTOMAONETISJL 



PHOTOMETER 



471 



In scientific photography much remains to be done. We know but 

 little of the properties of light in its influence on vegetation and 

 animal life. Mr. Robert Hunt and others have, however, established 

 some interesting facts in the former direction, and lately some experi- 

 ments mad* upon the eggs of insects sssni to show that light of various 

 colours and intensities acts differently according to its colour and other 

 peculiar qualities. There is no branch of science which will better 

 repay the philosophical experimentalist for bis investigations than that 

 of photography. The most marvellous and unexpected results have 

 been constantly obtained. 



Those who would pursue photography further should consult 

 Hunt's ' Researches on Light,' the Abb/ Moigno's Repertoire d'Op- 

 tique Modern* ,' and Mr. Hardwich's 'Treatise on Photographic 

 Chemistry.' There are many papers also of interest to be found in 

 the ' Comptes Rendus ' of the Paris Academy of Sciences, in our own 

 Royal Society's ' Transactions,' and above all in the journals of the 

 various Photographic Societies. 



PHOTOMAONETISM. A term sometimes applied to Dr. Faraday's 

 beautiful discovery, by which the phenomena of magnetism have been 

 placed in relation with those of light. [MAGNETISM.] 



PHOTOMETER (literally "light-measurer," from ?> and /irrfr), 

 the name given to instruments constructed for the purpose of measuring 

 the relative illuminating powers of different sources of light When 

 light or heat falls upon any substance, it is disposed of either by 

 reflection, absorption, or transmission, or else by two of them, or all 

 three of them combined. If two" substances could be found which 

 would reflect, absorb, and transmit calorific rays with the same inten- 

 sity, and likewise reflect luminous rays equally, but differ in their 

 powers of absorbing and transmitting light, we should then possess the 

 means of at least ascertaining whether the absorption of light alone 

 will produce effects analogous to what is observed to follow the absorp- 

 tion of heat. For this purpose it would be only necessary to prepare 

 a differential thermometer whose bulbs were of the substances possess- 

 ing the properties alluded to. The calorific rays accompanying the 

 incident light would, by acting equally upon the two bulbs, produce no 

 change in the indications of the instrument, and the only alteration, if 

 any, which could ensue, would arise from the unequal absorption of 

 light by the two bulbs. This alteration, however, when observed, 

 though it might be considered a correct measure of the quantity 

 absorbed, could not be taken for a measure of the quantity or 

 brightness of the incident light, unless it could be further shown that 

 the quantity absorbed by the same substance is proportional to the 

 quantity of incident light, whatever may be its nature, that is, whether 

 it be solar light, gas light, ic. 



The photometer invented by Leslie differs from the instrument we 

 have supposed, merely in its being in some respects leas deserving of 

 the name. It consists of a differential thermometer having one of ita 

 bulbs of plain transparent glass, the other of the same material coated 

 either with Indian-ink or black enamel. Leslie remarks : " The rays 

 which fall on the clear ball pass through it without suffering obstruc- 

 tion ; but those which strike the dark ball are stopped and absorbed at 

 ita surface, where, assuming a latent form, they act as heat. This heat 

 will continue to accumulate till its further increase comes to be 

 counterbalanced by an opposite dispersion, caused by the rise of 

 temperature which the ball has come to acquire. At the point of 

 equilibrium therefore the constant accessions of heat derived from the 

 action of the incident light are exactly equalled by the corresponding 

 portions of it again abstracted in the subsequent process of cooling. 

 But in still air the rate of cooling is, within moderate limits, propor- 

 tional to the excess of the temperature of the heated surface above 

 that of the surrounding medium. Hence the space through which the 

 coloured liquid sinks in the stem will measure the momentary impres- 

 sions of light, or ita actual intensity.'' Allowing that the light incident 

 upon the clear ball is wholly transmitted, and that that which strikes 

 the dark ball is wholly absorbed, assumes a latent form, and then acts 

 as beat, it by no means follows that the effect produced upon the 

 instrument was wholly or even chiefly attributable to the absorption 

 of light, since we learn from Leslie's own experiments that the 

 calorific rays which accompany the incident light would be more 

 abundantly absorbed by the dark than by the light ball. This has 

 sines been so satisfactorily established by the observations of 

 Thomson and others, tiiat, as a measurer of light, the instrument may 

 be regarded as useless. 



The defects of Leslie's photometer were to a considerable extent 

 obviated by the late Professor Ritchie, who, in 1825, communicated to 

 the Royal Society the description of a new photometer. In order to 

 intercept the calorific rays accompanying the light experimented upon, 

 he transmitted the Utter through a thick circular disc of glass into a 

 metallic air-tight cylinder, the diameter of which was considerable 

 compared with its depth. The axis of the cylinder was placed horizon- 

 tally, anJ the aperture covered by the glass was the only one through 

 which the light was admitted. Across the interior of the cylinder 

 was stretched a circular sheet of dark inner, which absorbed the trans- 

 mitted light, and, as was supposed, thereby converted it into heat, 

 which became sensible by its expanding the air within the cylinder. 

 A second cylinder of the same form and construction was placed by 

 the aide of the first so that the line of axes might coincide, but with 

 the aperture for the admission of light turned in the contrary direction, 



and in that position they were connected by a bent thermometer tube 

 containing a coloured fluid, which served to prevent the air of one 

 cylinder from mixing with that of the other. So long as the air in the 

 two cylinders possessed the same degree of elasticity, the level of the 

 fluid in the two branches of the tube was of course the same ; and a 

 variation of level indicated a variation in the elasticity of the two bulks 

 of sir, arising from the more energetic action of the medium admitted 

 through one aperture than through the other. To compare the relative 

 intensities of two lights, the instrument was placed anywhere between 

 them, and approached towards one or the other, until it was found that 

 the position of the fluid in the tube was the same as when the instru- 

 ment was not under the influence of the lights. Supposing the whole 

 of the calorific rays and none of the luminous rays to nave been inter- 

 cepted by the glass, this position determined the point at which the 

 intensity of the two lights was the same ; and hence, since the intensity 

 of light varies inversely as the square of the distance from its source 

 [LIQBT], it followed that at equal distances from their respective 

 sources their intensities were directly proportional to the squares of 

 their observed distances from the instrument. 



The same gentleman afterwards constructed a very simple instrument 

 which affords a good measure of the relative brightness of two lights, 

 provided they are of the same colour. The principle originated with 

 Bouguer, who published it in his ' Traitd d Optique,' in 1760. The 

 annexed figure represents a vertical section ol the instrument. It 



consists of a rectangular box open at both ends and blackened upon its 

 inner surface. On the top is a long narrow rectangular slip AB, 

 covered with tissue or oiled paper. Within are two sheets of plane 

 looking-glass, c D and c E, cut from the same slip to ensure uniformity 

 of reflection. Each sheet has the same width as the box, and its 

 length equal to the hypothenuse of a right angled isosceles triangle, 

 whose side is the height of the box. Their reflecting surfaces are 

 turned towards the open ends of the box, and their upper extremities 

 rest against each other along a line, which in the figure is projected 

 into the point c, and which divides the aperture A i> into two equal 

 parts, separated by a narrow strip of black card to prevent the mingling 

 of the lights reflected from the two planes. In using the instrument 

 it is placed between the lights whose intensities are to be compared, so 

 that they may be reflected from c D and c E upon the tissue paper A B. 

 It is then approached nearer to one or the other until, to an eye situated 

 above A B, the two portions A o and B c appear equally illuminated, 

 which, on account of the immediate proximity of A o and B c, may be 

 determined with tolerable correctness, the colour of the two lights 

 being supposed the same. The distances of the lights from the vertical 

 c F being measured and squared, give the direct ratio of the intensities. 



There is a mode of comparing the illuminating powers of two lights 

 suggested by Count Rumford, which is remarkable for the facility with 

 which it may be applied, and the simplicity of the requisite apparatus, 

 nothing more being needed than a smooth surface of small extent and 

 of a light uniform colour, and a blackened stick for throwing a shadow. 

 The surface is illuminated by the two lights experimented upon, which 

 are to be so placed, that when the stick is interposed between them and 

 the surface, the two shadows may be nearly in contact, which will 

 enable the eye to decide whether they are of equal depth, and will nt 

 the same time ensure the intercepting of rays equally inclined to the 

 surface. So long as the shadows are of unequal depth, one of the 

 lights must be brought nearer to or withdrawn farther from the 

 surface till an equality of depth is obtained, and then the squares of 

 the perpendicular distances of the lights from the surface give the 

 ratio of their intensities. If an equality between the inclination of 

 the intercepted rays to the surface cannot be obtained, then, when 

 the two shadows are of the same depth, the intensities of the lights 

 will be directly proportional to the squares of their perpendicular 

 distances from the surface, and inversely proportional to the sines of 

 the inclinations of the intercepted rays to the surface. 



Suppose, for example, it were required to ascertain the illuminating 

 power of a gas light burning & cubic feet of gas per hour, as OOOparM 

 with that of a sperm candle burning 132 grains of spermaceti per 

 hour ; suppose the screen to be at 100 inches from the gas-light, and 

 the candle only 2775 inches in order to equalise the shadows. The 

 relative intensities of the two lights are then found by squaring the 

 distances of each light from the screen ; the gas-light will diffuse a 

 light which bean the same proportion to that of a candle as 100-' : 

 277S 1 ; or as 16 to 1. 



Wheatatone's photometer consists of a small bright bead, to which 

 a rapid looped motion is imparted, and this being placed between the 

 two lights, both are seen reflected from the different points of the 

 bead's surfr.ce. By the principle of persistence of impressions on the 



