Prof. Magnus's Hydraulic Researches. 169 



seen in it. In order therefore to observe the different portions 

 of the jet, it was necessary to fasten the mirror with the clock- 

 work upon a stand which could be raised or lowered at will. 

 In the same manner the lamp was obliged to be fastened upon 

 another stand in order to illuminate the different portions of the 

 jet. 



143. When no note is sounded and all is in perfect tran- 

 quillity, if the strongly-illuminated smooth part of the stream be 

 viewed, a broad luminous surface is seen in the rotating mirror. 

 If, however, swellings are produced by means of the magnetic 

 hammer mentioned in § 125, and the mirror is adjusted to one 

 of them, isolated separate luminous lines are seen in the mirror 

 in the manner shown in fig. 5. 



1-14. These lines can only arise from the observed part of the 

 jet either consisting of separate rounded masses, or having an- 

 nular swellings — that is, alternating greater and smaller dia- 

 meters. For if we imagine a small round mass of water — a 

 drop — to remain in a fixed position, and the mirror in which 

 the drop is seen reflected also to i-emain still, we can only 

 see a luminous point, because the rounded mass of water only 

 reflects the light falling upon one point of its surface in such 

 a manner that it can pass from the mirror into the eye of the 

 observer. If the mirror remains stationary without revolving 

 while the drop is moving downwards, instead of a point, a 

 luminous vertical line is seen. But if the mirror turns round 

 while the drop is falling, the vertical line is changed into one 

 inclined to the horizon, and under certain circumstances into a 

 bent one, whose inclination is greater according as the mirror 

 rotates more quickly. But at the place at which the drop was 

 first seen, a second, then a third, &c. appears, each one forming 

 a similar inclined or bent line of light. As, however, the mirror 

 has already changed its position when the second drop comes 

 into the place of the preceding one, the different drops are 

 reflected sinmltaneously from different parts of the mirror, and 

 hence the lines appear to run together side by side. The di- 

 stance between these lines depends upon the angular velocity of 

 the miiTor. I have seen them most distinctly when the mirror 

 turns once in a second upon its axis. 



145. These lines may also be seen if the mirror be not set in 

 rotation by clockwork, but if it be moved backwards and forwards 

 by hand. In this case, however, the lines cross one another and 

 form a net-like figure, similar to fig. G. 



146. If the continuous portion of the stream above the swell- 

 ings be viewed by the mirror when swellings are ju'oduced by 

 Necf 's hammer, and remain very constant, lines similar to those 

 produced by the swellings are seen ; these, however, are much 



