170 Intelligence and Miscellaneous Articles. 



able sediments to that extent are found over those parts of the areas 

 first submerged and which remained undisturbed. That volcanic 

 action was chiefly confined to parts of the regions which became 

 first submerged ; that the immediate cause of these outbursts was 

 the weakness of the prae-Cambrian crust at those parts, from the 

 great depression that had taken place, it being too thin there 

 to resist the pressure from within, and to bear the weight of the 

 superincumbent mass of soft sediment. 



5. That the seat of volcanic action at this time was at a depth of 

 probably not less than 25 miles, as sediments which were depressed 

 to a depth of from 9 to 10 miles do not indicate that they had been 

 subjected to the effect of any great amount of heat, and are free 

 from metamorphosis. 



6. That the climate at the early part of Palaeozoic time was one 

 of very considerable, if not extreme cold, and that it became gra- 

 dually milder after each period of depression. That towards the 

 close of the Palaeozoic, in consequence of the elevation of very large 

 areas, and to a great height, the climate became again more rigo- 

 rous in character. 



7. That the various changes which took place over the northern 

 latitudes during Laurentian and Palaeozoic times allowed marine 

 and land life to develop and progress in those areas at interrupted 

 periods only ; consequently most of the progressive changes in the 

 life had to take place in more equatorial areas, where the sea-bottom 

 was less disturbed, and where the temperature was more equable. 

 Any imperfection therefore in the Palaeontological record belonging 

 to these early times should be attributed to these and like circum- 

 stances ; for wherever an approach to a complete record of any part 

 of the chain is preserved to us, the evidence points unmistakably to 

 an order of development, through a process of evolution from lower 

 to higher grades of life. 



XXI. Intelligence and Miscellaneous Articles. 



RECOMPOSITION OF THE COMPONENT COLOURS OF WHITE LIGHT. 

 To the Editors of the Philosophical Magazine and Journal * 



University College, Toronto, 

 Gentlemen, January 13, 1876. 



rpiLE following experiment to illustrate the recomposition of the 

 -*■ component colours of white light was devised by me about a 

 year ago; and as I have looked in vain for it in the scientific 

 periodicals and works on experimental physics with which I am 

 acquainted, I have thought it might be useful to some of your 

 readers. 



The spectrum of the light of the sun or of the electric lamp is 

 formed in the usual way, the slit being vertical ; and the spectrum 

 is projected on to the screen by means of a small plane mirror 

 placed close to the prism. If this mirror be capable of vibrating 

 through a small angle around a vertical axis, the vibrating spectrum 

 on the screen will give rise to a horizontal band of white light with 

 red and blue ends. 



The advantages of this method over that of the vibrating prism 



