no ELEMENTS OF SCIENCE 



much higher up the screen than it would have fallen but 

 for the prism. When a broad part of the prism was 

 downwards so that its sides approached each other from 

 below upwards, the beam was reflected downwards instead 

 of upwards. But change of place was by no means the 

 interesting point of the experiment. In the first place 

 the bright spot became, as before said, elongated, and 

 secondly it exhibited the hues of the rainbow the red 

 being lowest and the violet uppermost. This dis- 

 persion of the different coloured rays showed that they 

 are bent (refracted) unequally in passing through the 

 prism. The elongated coloured spot is called a spectrum. 

 The ordinary spectrum thus formed by the sun's light 

 called a solar spectrum may be said to be an elongated 

 image of the sun. But spectra may be formed by light 

 from other sources. It is always the violet rays which 

 are the most refrangible, and the red which are the least 

 so always and in every medium. Nevertheless the 

 ratio, or proportion, between the highest and lowest 

 degrees of refrangibility is different in different media, 

 (e.g., plate glass thus differs from flint glass), and this 

 enables the optician to construct what are called achro- 

 matic instruments. These are instruments purposely so 

 arranged as to do away with the disturbing effects of the 

 dispersion of colours which, without such aid, would take 

 place in optical instruments, and greatly mar their 

 utility. 



With the exception of bodies which are themselves 

 luminous, none can appear of any colour which does not 

 exist in the light they receive. Thus if only green light 

 be supplied to a red object, it will appear neither red nor 

 green, but perfectly black ; for the green rays will be 

 absorbed and not reflected as such. All distinctions of 

 colour, save differences of intensity, may be made to 



