NATURE 



Oct. 7, 1880] 



PHYSICS WITHOUT APPARATUS^ 

 VII. 



IN the preceding articles on "Physics without Appa- 

 ratus" it has been shown how a large proportion of 

 the fundamental experiments in most branches of physics 

 can be performed without employing expensive appa- 

 ratus. . , . . , 



The next of these branches to claim consideration is the 

 science of optics. Here again, as in electricity and in 

 heat, we find that, while the higher quantitative laws of 

 the science require for their experimental proof apparatus 



537 



of the finest and most exact and therefore most expensive 

 nature, the ekmenia/y facts of experiment are readily 

 demonstrable with little or no apparatus of a formal 

 kind. 



An ordinary looking-glass, a lighted taper, and a foot- 

 rule or a measuring tape are quite sufficient to demonstrate 

 the simple geometrical laws of reflection ; for with their 

 aid it is very easy to show that the image of the candle in 

 the mirror is virtually situated at a distance behind the 

 mirror equal to the actual distance of the candle in front, 

 and that, when a ray falls obliquely on the mirror, the 

 angle of incidence is equal to the angle of reflection. A 

 teacher who wishes to go further into the matter, and to 

 demonstrate the laws of reflection at curved surfaces, 

 usually provides himself with the appropriate silvered 

 mirrors of convex and concave form. Failing these, the 

 exterior and interior surfaces of the bowl of a bright 

 silver spoon will probably be as satisfactory a substitute 

 as any. We have found even a saucer of common glazed 

 earthenware to form a very fair concave mirror, giving 

 upon a small tissue paper screen a beautiful little inverted 

 image of a distant gas flame. 



To illustrate the geometrical laws of refraction through 

 lenses,'a good reading-glass of large size is a desirable 

 acquisition. Spectacle-lenses, though of smaller size, 

 and therefore admitting less light, are also of service. In 

 the absence of any of these articles, it is generally possible 

 to fall back upon a water-decanter, provided one can be 

 found of a good globular form, and not spoiled for optical 

 purposes by having ornamental work cut upon the sides 

 of the globe. Fi^. 22 shows how this decanter, filled with 

 water, is to be employed. It is held a few inches away 

 from a white wall, and a candle is placed at the opposite 

 side, so that its light falls through the decanter on to the 

 wall. The candle is moved towards or away from the 



' Continued from p. ^09, 



decanter until the position is found in which its rays focus 

 themselves upon the wall giving a clear inverted image of 

 the candle flame upon the wall. The experiment may be 

 varied by setting down the candle on the table and then 

 moving the decanter to and fro until a definite image is 

 obtained. If a large hand reading-glass be available, the 

 image will be much clearer than with the improvised 

 water-lens ; and a further improvement in the manner of 

 experimenting maybe made by using a screen of white 

 paper or card instead of a whitened wall on which to 

 receive the image. The sheet of paper should be set up 

 in the simple fashion shown in Fig. 24, at one end of a 

 table. The candle should be placed at the 

 other end of the table, and the reading-lens 

 moved about between them until a point is 

 found at which it throws upon the screen a 

 good clear image of the candle. It will be 

 found that there are two such points, one 

 near the candle, the other near the screen. 

 In each case the image of the candle will 

 be inverted, but in the first case it will be a 

 magnified and in the second a diminished 

 image, the size of the image, as compared 

 with that of the real flame, being proportional 

 to their respective distances from the lens. 

 When the lens has been placed in a position 

 of good focus, the candle may be removed 

 and placed where the screen stood ; if now 

 the screen is placed where the candle was, 

 it will be found that the image is again visible 

 on the screen, still inverted, though altered 

 in magnitude. This experiment, in fact, 

 proves the law of conjugate foci. 



The young beginner in science who re- 

 peats these experiments for himself will begin 

 to understand how it is that in the photo- 

 grapher's camera the image in the instrument 

 is inverted, and how it can also be true that 

 the images cast on the sensitive retina of the 

 eye arc also inverted. The retina at the back of the eye- 





Fl3. 23. 



ball answers to the white screen on to which the image is 



