:o\ 



NA TURE 



[June 28, 1883 



the left you have the wave-length of light in fractions of 

 a centimetre ; the unit in which these numbers to the left 

 is measured is the 1/100,000 (or 10-5) of a centimetre. 

 We have then, of visible light, wave-lengths from ~]\ to 4 

 nearly, or 3*9. You may say then roundly, that for the 

 wave-lengths of visible light, which alone is what is re- 

 presented on that table, we have wave-lengths of from 4 

 to 8 on our scale of 1/100,000 of a centimetre. The 8 is 

 invisible radiation a little below the red end of the spec- 

 trum. The lowest, marked by Fraunhofer with the letter 

 A, has for wave-length 7^/100,000 of a centimetre. On 

 the model before you I will now show you what is meant 

 by a "wave-length;" it is not length along the crest, 

 such as we sometimes see well marked in a breaking 

 wave of the sea, on a long straight beach; it is distance 

 from crest to crest of the waves. [This was illustrated 

 by a large number of horizontal rods of wood connected 

 together and suspended bifilarly by two threads in the 

 centre hanging from the ceiling ; T on moving the lower- 

 most ro1, a wave was propagated up the series.] Imagine 

 the ends of those rods to represent particles. The rods 

 themselves let us suppose to be invisible, and merely 

 their ends visible, to represent the particles acting upon 

 one another mutually with elastic force, as if of india- 

 rubber bands, or steel spiral springs, or jelly, or elastic 

 material of some kind. They do act on one another in 

 this model through the central mounting. Here again is 

 another model illustrating waves (Fig. z). 2 The white 

 circles on the wooden rods represent pieces of matter — 

 I will not say molecules at present, though we shall deal 

 with them as molecules afterwards. Light consists of 

 vibrations transverse to the line of propagation, just as in 

 the models before you. 



1 The details of this bitilar suspension need not be minutely described, 

 as the new form, with a single steel pianoforte w.re to give the required 

 mutual forces, described belcw and represented in Fig. 2, is betier and more 

 tasilv made. 



2 This apparatus, which is represented in the woodcut. Fig. a, is of the 

 following dimensions and description. The series of equal and similar bars 

 (b) of which the ends represent molecules of the medium, and the pendulum 

 bar (p), which performs the part of exciter of vibrations, or of kinetic i-tore 

 of vibrational energy, are pieces of wood each 50 centimetres long, 3 centi- 

 metres broad, and 1*5 centimetres thick. The suspending wire is steel 

 p.anoforte wire No. 22 B W.G. ('07 of a cm. diameter), and the bars are 

 secured to it in the folluwing manner. Three brass pins of about "4 of a 

 centimetre diameter are fitted loosely in each bar in the position as indi- 

 cated ; i.e. forming the corners of an isosceles triangular figure, with its base 

 parallel to the line of the suspending wire, and about 1 mm. to one side of 

 it. The suspending wire, which is laid in grooves cut in the pins, is passed 

 under the upper pin, outside the pin at the apex of the triangle, over the 

 upper side of the lower pin, and thence down to the next bar. The upper 

 end of this wire is secured by being taken through a hole in the supporting 

 beam and several turns of it put round a pin placed on one side of the hole, 

 as indicated in the diagram. To each end of the pendulum bar is made fast 

 a steel spiral spring as shown ; the upper ends cf these spiings being secured 

 to short cords wh.ch pass up through holes in the supporting beam, and are 

 fastened by two or three turns taken round the pins. These steel springs 

 serve as potential st res of vibrational energy alternating in each vibration 

 with the kinetic store constituted by the pendulum bar. The ends &f the 

 vibrating bars (d) are baded with masses of lead attached to them. The 

 much larger masses of lead seen on the pendulum bar, which are adjustable 

 to different positions on the bar, are, in the diagram, shown at the smallest 

 distance apart. The lowermost bar carries two vanes of tin projecting down- 

 wards, which dip into viscous liquid (treacle diluted with water) contained 

 bn the vessel (c). A heavy weight resting on the bottom of this vessel, and 

 connected to the lower end of the suspending wire by a stretched indiarubber 

 band, serves t'> keep the lower end of the apparatus in position. The period 

 of vibration of the pendulum bar is adjustable to any desired magnitude by 

 shift. ng in or out the attached weights, or by tightening or relaxing the 

 cords which pull the upper ends of the spiral springs. 



Now in that beautiful experiment well known as 

 Newton's rings we have at once a measure of length in 



U^~ 



U * 



I ~^-~- 



^A3 



Z^B b 



3 



r ----" LP B 







^m 



U B 



~D B 



=^~0 



B 



the distance between two pieces of glass to give any par- 



