130 



- KNOWLEDGE ♦ 



[Aug. 15, 1884. 



sensibly equidistant on each side of the lens (and we must 

 shift them about until they are), then, if we take the 

 distance between them very accurately — preferably with 

 compasses and a finely-divided scale — this, as before, will 

 be four times the focal length of the lens for parallel rays. 

 And while on the subject of the measurement of the focus 

 of lenses, we may just mention a simple way in which that 

 of an equi-concave lens may be ascertained ; a kind of deter- 

 mination which sometimes puzzles the beginner. Blacken 

 one side of the lens whose focus you wish to find, and draw 

 a diametrical line across the black backing. On this 

 line make two dots, about the twentieth of an inch in 

 diameter, and equidistant from the centre (or, what is of 

 course the same thing, from the edges) of the lens. Now, 

 hold your lens square to the Sun's rays with its unblacked 

 face turned towards him, and place a card screen, as nearly 

 parallel as may be to the face of the lens to receive the 

 resulting image. Measure very carefully the distance apart 

 of the spots on your lens, and shift the screen about until 

 their images are exactly twice this distance apart ; then 

 will the distance between the screen and the lens be the 

 \"irtual focus of the latter. 



While we are on the subject of measurement, and 

 before proceeding to the consideration of the structure 

 of the human eye, and of the phenomena of xii'ion, 

 we may say something on the very interesting subject 

 of photometr}', or the measurement of the relative amounts 

 of light emitted by various sources of it. We see lamps 

 advertised which are guaranteed to give the light of 

 sixteen or twenty candles, as the case may be. How can 

 we find out for ourselves whether this quantity of light 

 really is emitted by any given flame or not 1 The principle 

 on which this is done will be apparent from Fig. 2.3, 



Fig. 23. 



wherein C is the flame of a candle, from which rays 

 CR, CRi, &c., are radiating. If now we place a piece 

 of cardboard, S, one foot square, at S, a distance of 2 ft. 

 from the candle, C, a glance at our figure will show that 

 its shadow will just cover an area of four square feet (i.e. 

 two feet high and two feet wide), and so on. Or, 

 putting it in another way, if we removed the first screen, 

 S, altogether, the light falling on an equal area of another 

 screen, S', at twice the distance would only be one-quarter 

 as intense as that lighting the nearer screen. At three 

 times the distance, S", the candle would only give one-niuth 

 of the light it does at distance unity ; upon an equal area 

 four times as far ofl', one-sixteenth, and so on. And this 

 we may express in the form of the foUowiug law : the 

 intensity of light is inversely proportional to the square of 

 the distance of the illuminated surface from the source of 

 it. Hence a very little thought will show that when two 

 sources of light produce equal iutensities of illumination at 

 different distances, their illuminating powers must be in the 



ratio of the squares of their distances from the illuminated 

 surlace. This supplies us with a ready means of measuring 

 the intensity of any given light. For we may adopt 

 llumford's method, illustrated in Fig. 24. 



Fig. 24. 



In this figure S represents a white card screen, in front of 

 which stands a rod about as thick as an ordinary drawing 

 pencil. It will be seen that each of the two lights to be 

 compared casts a shadow of the rod, and each light illumi- 

 nates the shadow cast by the other. Let us suppose now 

 that our candle, C, is one foot from the screen ; then if, on 

 moving the lamp, L, to a distance of four feet from the 

 screen, the two shadows appear identical in depth, the 

 lamp really must give sixteen times the light of the candle. 

 It will be quite evident that had it given a precisely similar 

 shadow to that caused by the candle when removed to a 

 distance of five feet, it must have emitted light of twenty- 

 five-candle power, and so on. Bunsen's photometer is, if 

 possible, more simple still in construction, consisting, as it 

 does, mainly of a sheet of white paper with a grease spot on 

 it. This, of course, lets more light through it than the rest 

 of the paper. Hence, if it be illuminated more strongly 

 from behind, it will appear to be bright on a dark ground. 

 If the front surface be the more strongly lighted, it will 

 seem to be dark upon a light ground ; while, if absolutely 

 equally illuminated on both sides, it will disappear as a 

 spot, and be merged in the general surface of the paper. 

 In employing this simple device a sheet of white paper 

 should be stretched on a frame, and the lights, whose rela- 

 tive intensity is to be measured, placed one on each side of it 

 at the height of the spot ; then they are moved about until the 

 spot disappears, and, such disappearance being complete, 

 the distances of the lights are measured from the screen, 

 the squaring of those distances as before giving the com- 

 parative intensities of the lights. One practical word of 

 caution is needed here as to the method of making the 

 grease-spot, as oil would create one of which it would be 

 very difficult to cause the disappearance. The best plan is 

 to let a drop or two of stearine fall on to the paper from a 

 burning candle, and when it has cooled scrape the super- 

 fluous stearine ofi" the paper with a pen-knife. Then the 

 stearined spot should be placed in a fold or two of blotting- 

 paper, and ironed with a hot iron until it becomes of the 

 proper strength. This the reader must ascertain for him- 

 self by direct experiment. The two common forms of 

 photometer which we have just described wUl amply serve 

 to illustrate the principle on which such measurements are 

 made, and will afford the student both amusement and 

 instruction. Into the great subject of celestial photometry 

 we cannot here enter, our object being, as we have before 

 stated, to describe optical experiments which can be per- 

 formed without the aid of any elaborate or costly apparatus, 

 and with materials within the reach of everybody. 



The Medical Press and Circular savs : — One of the effects of the 

 cholera scare is the sudden increase in market prices of all kinds of 

 disinfectants. Opium, too, has gone np twenty-five per cent., and 

 sulphate of morphine has likewise advanced. We hear that the 

 demand is so great from France that one well-known English 

 house cannot manufacture carbolic acid sufficiently fast to meet it. 



