100 ANNUAL OF SCIENTIFIC DISCOVERY. 



THE TENEBROSCOPE FOR PROVING THE INVISIBILITY OF LIGHT. 



At the last meeting of the British Association, the Abbe Moigno 

 exhibited and described an instrument invented by M. Soieil, of 

 Paris, for illustrating the invisibility of light, and called the " Teuebros- 

 cope." It is well known to scientific men, although the general public 

 do not sufficiently appreciate the fact, that light in itself is invisible 

 unless the eye be so placed as to receive the rays as they approach it, 

 or unless some object be placed in its course, from whose surface the 

 light may be reflected to the eye, which will generally thus give notice 

 of the presence of that object. Thus, if a strong beam of sunlight 

 be admitted into a darkened chamber through a small opening, and re- 

 ceived on some blackened surface placed against the opposite wall, the 

 entire chamber will remain in perfect darkness, and all the objects in 

 it invisible, except in as far as small motes floating in the air mark the 

 course of the sunbeam by reflecting portions of its light. Upon pro- 

 jecting a fluid or small dust across the course of the beam its presence 

 also becomes perceptible. The instrument exhibited consisted of a 

 tube with an opening at one end to be looked into, the other end 

 closed, the inside well blackened, and a wide opening across the tube 

 to admit strong light to pass only across. On looking in, all is perfect- 

 ly dark, but a small trigger raises at pleasure a small ivory ball into 

 the course of the rays, and its presence instantly reveals the existence 

 of the crossing beam by reflecting a portion of its light. 



THE STAR CHROMATOSCOPE. BY M. CLAUDET. 

 The scintillation and change of colors observed in lookino- at the 



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stars are so rapid that it is very difficult to judge of the separate 

 lengths of their duration. If we could increase on the retina the 

 length of the sensations they produce, we should have the better means 

 of examining them. This can be done by taking advantage of the 

 power by which the retina can retain the sensation of light during a 

 fraction of time which has been found to be one-third of a second a 

 phenomenon which is exemplified by the curious experiment of a piece 

 of incandescent charcoal revolving round a centre, and forming a con- 

 tinual circle of light. It is obvious that if the incandescent charcoal, 

 during its revolution, was evolving successively various rays, we could 

 measure the length and duration of every ray by the angle each would 

 subtend during its course. This is precisely what can be done with 

 the lio;ht of the star. It can indeed be made to revolve like the incan- 



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descent charcoal, and form a complete circle on the retina. When we 

 look at a star with a telescope, we see it on a definite part of the field 

 of the glass ; but if with one hand we slightly move the telescope, the 

 image of the star changes its position, and during that motion, on ac- 

 count of the persistence of sensation on the retina, instead of appear- 

 ing like a spot, it assumes the shape of a continued line. Now if, in- 

 stead of moving the telescope in a straight line, we endeavor to move 

 it in a circular direction, the star appears like a circle, but very irregu- 

 lar, on account of the unsteadiness of the movement communicated by 

 the hand. Such is the principle of the instrument employed by the 

 author to communicate the perfect circular motion which it is impossi- 

 ble to impart by the hand. The instrument consists of a conical tube, 



