I 



Jan. I, 1874J 



NA TURE 



169 



mately parallel to the incident ray. Fig. 12 shows an 

 end view of a Nicol's prism, the shorter diagonal in the 

 direction of vibration of the emergent polarised ray. 



Two such instruments, when used together, are respec- 

 tively called the "polariser" and the "analyser," on account 

 of the purposes to which they are put. These, when placed 

 in the path of a beam of light, give rise to the following 

 phenomena, which are, in fact, merely a reproduction in 

 a simplified fonii of what has gone before. 



When polariser and analyser are placed in front of 

 one another, with their shorter diagonals parallel, that 

 is, when the vibrations in the image transmitted by the 

 one are parallel to those in the image transmitted by the 

 other, the light will be projected on the screen e.xactly as 

 if only one instrument existed. If, however, one instru- 

 ment, say the analyser, be turned round, the light will be 

 seen to fade in the same way as in the case of the tour- 

 malin plates ; until, when it has been turned through a right 

 angle, or as it is usually expressed, when the polariser 

 and analyser are crossed, the light is totally extinguished. 



In the complete apparatus or polariscope, we may 

 incorporate any system of lenses, so that we may 



make use of either parallel or convergent light, and 

 finally focus the image produced upon the screen or 

 upon'the retina. At present we shall speak only of the 

 phenomena of colour produced by crystal plates in a 

 parallel beam of polarised light — chromatic polarisation, 

 as it is called, with parallel light. 



Various forms of polariscopes have been devised, whereof 

 the three described below may be regarded as the most 

 important. 



Fig. 13 is an elevation of one of them. When used in 

 its simplest form, the frame F carries a plate of black 

 glass which is capable of revolving about pivots in the 

 uprights. The positions of the source of light and of the 

 frame must be adjusted so that the plate will receive the 

 incident light at the polarising angle, and reflect it in 

 the direction of the eye-piece which contains a Nicol or 

 other analyser. The objects to be examined are to be 

 placed on the diaphragm E. 



This instrument may be converted into another form, 

 due to Norremberg, by placing a silvered mirror horizon- 

 tally at H. The plate of black glass must be removed 

 from the frame F, and a plate of transparent glass substi- 

 tuted for it, which inust be so inclined that the light fall- 

 ing upon it shall be reflected at the polarising angle per- 



pendicularly towards the horizontal mirror. The object 

 may be placed on the diaphragm E as before. But it may 

 also be placed on the diaphragm D below the polarising 

 plate F, and in that case the eye will receive the polarised 

 ray reflected from the mirror ; and the polarised ray will 

 have passed, before it reaches the eye, twice through the 

 crystalline plate placed between the mirror and the 

 polariser. The result is the same as if, in the ordinary 

 apparatus, the polarised ray had passed through a plate 

 of double the thickness. If the plate does not fill the en- 

 tire field of view two images of the plate will be seen, the 

 one larger, as viewed directly, the other smaller, as viewed 

 after reflection from the horizontal mirror ; the first will 

 show the tint due to the actual thickness of the ci7stal, 

 the other that due to a plate of the same ci-ystal, but of 

 double the thickness. 



A further modification of this instrument will be de- 

 scribed hereafter. 



W. Spottiswoode 



(To be continued^ 



GALILEO'S WORK IN ACOUSTICS 



TN looking through the "Dialoghi delleNuove Scienze" 

 -'■ of Galileo, I came unexpectedly on a passage * con- 

 taining two remarkable discoveries in acoustics, which I 

 should have confidently referrred to a much later age. 

 For the sake of such of your readers as may share the 

 same erroneous impression, 1 hope you will allow me to 

 give, in Nature, a short account of these results. 



The first is a perfectly accurate explanation of the phe- 

 nomenon called "resonance." Every pendulum has a fi.xed 

 period of oscillation peculiar to itself. Even when the 

 " bob " is of considerable weight it is possible to set it 

 swinging through a large arc by merely blowing against it 

 with the mouth, provided the successive puffs arc properly 

 timed with reference to the pendulum's period of vibra- 

 tion. In the same way a single ringer can, by regular 

 pulling, throw the heaviest bell into oscillations of such 

 extent as to be capable of lifting half-a-dozen men who 

 should hang on to its rope, off the ground all together. 

 When a string of a musical instrument is struck, its vibra- 

 tions set the air in its vicinity trembling, and the tremors 

 thus set up spread themselves out through space. If they 

 fall on a second wire in unison with the first, and there- 

 fore prepared to execute its vibrations in the same 

 period, the effects of the successive impulses are accumu- 

 lated, and the wire's oscillations can be distinctly seen to 

 go on dilating until they have attained an extent equal to 

 those of the wire originally struck. 



Anyone who looks into the chapter on resonance in the 

 " Tonempfindungen '' will sec that the account of the phe- 

 nomenon given by the greatest living acoustician is, in 

 principle, identical with that of Galileo. 



The second point to which 1 wish to draw attention is 

 an experiment involving the earliest direct determination 

 of a vibration-ratio for a known musical interval. Galileo 

 relates that he was one day engaged in scraping a brass 

 plate with an iron chisel, in order to remove some spots 

 from it, and noticed that the passage of the chisel across 

 the plate was sometimes accompanied by a shrill whistling 

 sound. On looking closely at the plate, he found that the 

 chisel had left on its surface a long row of indentations 

 parallel to each other and separated by exacily equal 

 intervals. This occurred only when the sound was heard : 

 if the chisel traversed the surface silently, not a trace of 

 the markings remained. It was found that a rapid passage 

 of the chisel gave rise to a more acute, a slower to a less 

 acute, sound, and that, in the former case, the resulting 

 indentations were closer together than they were in the 

 latter. After repeated trials two sets of markings were 

 obtained which corresponded to a pair of notes making 



* Opere complete di Galileo Galilei. Vol. xiii. pp. 97-110. (Firenze.) 



