IIARDWICKE'S SCIENCE-GOSSIP. 



2 5 



THE TWO MIRRORS. 



By W. J. N. 

 No. IV. 



O complete the 

 solution of the 

 problem proposed 

 in the last article, 

 we have still to 

 determine the 

 position of the 

 bull's-eye conden- 

 ser, with which 

 we are to form the 

 pencil of parallel 

 rays. Accuracy 

 in this particular 

 is an important 

 condition of suc- 

 cess. If the rays 

 of the lamp be 

 not rendered truly 

 parallel, the focal 

 distance already 

 assigned to the mirror will be incorrect. 



The two " principal " foci of the bull's-eye, that is, 

 its foci for parallel rays falling either upon its convex 

 or its plane surface, must first be ascertained. 



Place the lamp at some distance from a wall of the 

 room. Near the wall-paper set the bull's-eye, on a 

 level with the lamp-flame, and having its plane side 

 turned towards the wall. Move it from and towards 

 the wall, until a small spot of light is seen in sharpest 

 focus on the wall paper, surrounded by a broad 

 margin of weak light. Measure the distance between 

 the plane side and the wall, and note it down as the 

 principal focal distance of the lens when the parallel 

 rays fall upon its convex surface. 



Next, turn the plane surface towards the lamp, 

 and repeat the experiment. The focal distance is 

 now increased, and the bull's-eye has to be set a little 

 further from the wall than before. The spot of light 

 formed at the focus is not so bright, but the margin 

 of diffused light has almost disappeared. Measure 

 the distance between convex surface and wall, and 

 No. 266. — February 1S87. 



note it down in the "principal focus," when the 

 parallel rays are received upon its plane surface. 



Now let us suppose the conditions of these ex- 

 periments to be reversed ; a luminous body, of the 

 same size as the focal spot of light, being placed at 

 the same distance from the same surface of the 

 bull's-eye as that measured in each of the two cases. 

 The divergent rays emanating from that body will be 

 rendered parallel by the bull's-eye. It appears, 

 therefore, that when we employ the bull's-eye to 

 reduce divergent rays to parallelism, either of its 

 two surfaces maybe turned towards the light; but 

 that there is a difference of focal distance in the two 

 cases, which must be remembered and attended to. 



When brilliancy of illumination is the first con- 

 sideration — as when using the spot lens — there will 

 be an advantage gained by placing the bull's-eye in 

 the nearer of the two positions, that is, with its plane 

 surface turned towards the light. In all other cases, 

 it is better to use the reversed position, the light 

 obtained being somewhat more pure, although a 

 little less powerful. 



In connection with this use of the bull's-eye, there 

 are two difficulties to be overcome. Firstly, the lens 

 has a natural infirmity, called spherical aberration. 

 Parallel rays passing near the margin are refracted 

 differently from those passing near the centre, and 

 come to a focus at a different distance. This caused 

 the border of weak light surrounding the image on 

 the wall-paper, above referred to. Marginal rays 

 had come to a focus between the wall and bull's-eye, 

 and then, crossing, had spread out so as to form the 

 illuminated border. This effect of spherical aberra- 

 tion, it will be remembered, was much less noticeable 

 when the convex side of the bull's-eye was turned 

 towards the wall. Conversely, when diverging rays 

 from a spot of light fall on the bull's-eye at the 

 distance of its principal focus, the marginal rays 

 instead of being rendered parallel, like the rest, are 

 rendered convergent. 



The second difficulty arises from the fact, that a 



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