428 The Hon. J. W. Strutt on an Electromagnetic Experiment. 



increase the focal distance by supposed " projection " on the 

 plane M N illusory, and will only add that, while all similar vague 

 theories of te projection" or out-seing are begging the question 

 when the question is of vision, in this case it is obvious that as 

 the plane M N was removed further off, the image thus projected 

 would increase in size; and if the projection represented the ap- 

 pearance as proposed, the pin would appear larger : but it proves 

 otherwise. 



It may be noticed that this analysis demonstrates the peculiar 

 effect of what I call single-ray delineation on the retina, which 

 will not only happen when the radiant point is behind the object, 

 showing it as a shadow, but also when an object, itself reflecting 

 rays, is seen through a pinhole. The effect in both cases is faint 

 vision, and an uninverted image on the retina, which would result 

 in both cases in inverted vision, save that in the latter there has 

 been a previous inversion which sets things right ; so that if, 

 while we wonder at the result of this " illusion," we are not sur- 

 prised at an object seen through a pinhole looking upright, it is 

 not from knowing more of what takes place. 



Your obliged Servant, 

 Rugby School, J. L. TuPPER. 



March 18, 1869. 



Note. — A spark at the end of a smouldering thread I have 

 found to exhibit the phenomenon most perfectly. Multiply 

 these sparks, and you multiply the images of the pin ; and the 

 same if you multiply the pinholes in the card E, their local rela- 

 tions appearing the same as in nature. 



LXII. On an Electromagnetic Experiment. By the Hon. J. W. 

 Strutt, Fellow of Trinity College, Cambridge** 



nr^HE experiment referred to is one described in the Philoso- 

 -t- phical Magazine for July 1869, p. 9, where it was shown 

 that, within certain limits, the magnetizing effect of a break-in- 

 duced current on steel needles is greater the smaller the number 

 of turns of which the secondary circuit consists, the opposite, of 

 course, being true of the effect on a galvanometer. The ground 

 of the distinction is that the galvanometer takes account of the 

 induced transient current as a whole ; while the magnetizing- 

 power depends mainly on the magnitude of the current at the 

 first moment of its formation, without regard to the time which 

 it takes to subside. 



But even with this explanation, few, I imagine, would be pre- 

 pared for the result who had not been accustomed to look at 

 * Communicated by the Author. 



