150 



KNOWLEDGE 



[July 1, 1895. 



telescope begins to move in the opposite direction. This 

 is the position of minimum deviation, and any motion of 

 the prism in either direction will increase the deviation. 

 The telescope is now clamped in its position and the 

 reading of the vernier on the graduated circle taken. The 

 direct reading of the slit is taken by turning the telescope 

 to view it when the intervening prism is removed. The 

 difl'erence between this last reading and the former is the 

 angle of minimum deviation. Observations are generally 

 made with the prism in the minimum deviation position, 

 for this position is a readily recoverable one, and obser- 

 vations at different times and in different instruments can 

 then be compared with each other. 



The distance between the red end and the violet end of 

 the spectrum is called the dispersion, and this depends on 

 the nature of the material forming the prism. The 

 amount of minimum deviation for a given ray depends on 

 the index of refraction of the material of the prism. 



When monochromatic light is used to illuminate the 

 slit — for example, the yellow light given out by a spirit-lamp 

 with a salted wick, or by a Bunsen burner in which a lump 

 of salt is heated — instead of a wide coloured spectrum, a 

 narrow image of the slit is obtained, of one colour only, 

 which would be yellow in the above instance. By observing 

 the minimum deviation in this case, we obtain its value for 

 certain yellow rays, and when a prism of another substance 

 is employed, a different deviation for these same rays is got, 

 and hence, when the angles of the prisms are known, the 

 indices of refraction of the different substances can be 

 compared. The refraction caused in light when it passes 

 through a prism is due to the difference of the velocity of 

 light through air, and through the substance of the trans- 

 parent medium forming the prism. If this medium consists 

 of a block with parallel faces, and the light falls per- 

 pendicularly upon it, retardation merely ensues, the light 

 proceeding in the same line after passing through as before, 

 but if the light falls obliquely it is bent in the transparent 

 slab at an angle to its first direction, and on passing out 

 proceeds in a line parallel with its original direction, but 

 displaced through a certain distance, depending on the 

 thickness and the refracting power of the slab. When 

 the refracting substance has faces inclined at an angle, 

 bending of the rays occurs both on entering and leaving it, 

 and we have the phenomena observed with prisms. It is 

 the rays which have the shortest wave-length whose velocity 

 is most altered in the prism ; these are the rays of 

 violet light. Thus violet light is most retarded— that 

 is, it is the most refrangible, its rays being the furthest 

 deflected from their original direction. Of the rays 

 forming the visible spectrum, those possessing the longest 

 wave-length and forming red light are the least bent. 

 It is pretty certain that the velocity of all the rays 

 from the violet to the red is the same in the free ether 

 of space. Now the velocity is equal to the product of the 

 frequency (or the number of vibrations per second) and 

 the wave-length ; therefore, these two quantities, which 

 remain unaltered during the passage of light through space, 

 ahange, one or both of them on the passage of the 

 light through dense matter. As the vibration in the 

 transparent medium is excited by that in the incident 

 light, its period is likely to be the same, so that it is 

 probably the wave-length and not the frequency which 

 changes as the light passes through the prism. Experi- 

 ment shows that the wave-length of red light at one end 

 of the \a8ible spectrum is about twice as great as that of 

 violet light at the other. The range of the vibrations to 

 which our eyes are sensitive is thus about an octave. The 

 red waves go through nearly four hundred millions of 

 millions of vibrations per second, while the violet vibrate 



about seven hundred and sixty million million times per 

 second — that is, about twice as fast. 



Solids and liquids, when raised to such a temperature 

 that they become white hot and luminous, give as a rule 

 a continuous spectrum — that is, one in which all the 

 visible rays are represented, from the dark red, right on 

 through the spectrum to the extreme violet. Gases, on the 

 other hand, when heated to incandescence and viewed 

 through a spectroscope, exhibit a spectrum which is made 

 up of a few or many bright lines on a dark background. 

 The number and position of these lines depend on the 

 nature and condition of the gas. One of the simplest cases 

 of gas spectra is that referred to above, obtained from 

 the salted flame. Here the incandescent gas is the vapour 

 of the element sodium, and at ordinary pressures, and at 

 the temperature of the Bunsen flame, its spectrum consists 

 of two yellow lines very close together. If the resolving 

 power of the spectroscope is small, these may appear as 

 one line. By special methods of investigation, Mr. 

 Michelson in America has shown that each of these yellow 

 lines in the spectrum of sodium is made up of several very 

 close together, but under all ordinary observation they 

 appear as two, or one double line. 



It can be readily understood why the spectrum of a gas 

 differs from that of glowing solids, for a gas consists of 

 molecules existing apart from each other and with a 

 distance between, these molecules being in rapid motion 

 and frequently coming into collision, but during the greater 

 part of their course being free from contact with their 

 neighbours. Not only does the motion of the gas particles 

 consist in a movement of translation from place to place, 

 but each has also a vibratory motion of its own, and the 

 constituent atoms composing the molecule have also 

 probably relative motions. It is these vibrations of the 

 molecule which, by communication of energy to the ether, 

 and thence to our eyes, give rise to the luminosity of the gas. 

 Now the different varieties of molecule have modes and 

 frequencies of vibration of their own, which they tend to 

 assume when shaken up and disturbed by collisions with 

 their neighbours. The only time during which they can 

 uninterruptedly vibrate in these characteristic modes ia 

 when they are on their "free path " — that is, on their course 

 between two collisions. In gases this time is much longer 

 than the time during which they are in collision, while in 

 solids or liquids the time during which a molecule is free 

 from contact with its neighbours is so small as to be 

 inappreciable. The molecules are practically always so 

 disturbed by jostling with their neighbours that they give 

 out vibrations of all sorts, and thus the rays of light from 

 strongly heated solids consist of all possible wave-lengths, 

 and a continuous spectrum is formed. 



SCORPIONS AND THEIR ANTIQUITY. 



By K. Lydekker, B.A.Cantab., F.R.S. 



TO the circumstance that scorpions have their bodies 

 protected by a coat of the hard substance techni- 

 cally known as chitine, the palteontologist is 

 indebted for a knowledge of their past history and 

 extreme antiquity ; and it is owing to the preserva- 

 tion of their remains in the Paheozoic strata of both the 

 old and new worlds that we are enabled to explain their 

 present geographical distribution. There are many other 

 groups of Invertebrates that we can have little doubt are 

 fully as ancient as scorpions, but which lack a hard 

 external investment, and whose past history is accordingly 

 a blank. One of the most remarkable instances of this is 

 afforded by the peculiar creatures termed Ffrlpatus, repre^ 



