152 PROCTOR, ON LIGHT. 
being led into another question and another ; each step opens 
out new paths on every side, all inviting us to a journey of 
discovery. I began by asking, Is every white body trans- 
parent? But where shallI end? As white as fine linen, flour, 
chalk, as white as snow. They are all poor comparisons, dull 
examples, nothing to be compared to many of the chemical 
precipitates. Precipitated chalk far outshines the natural 
varieties, and fine qualities of carbonate magnesia outshine it. 
Of a great number of substances I have compared, Howard’s 
carbonate of magnesia is the whitest, and microscopical exami- 
nation indicates that it consists of clear, colourless particles, 
but very minute. White lead consists of particles equally 
minute, and also transparent, but of a yellow-brown colour 
by transmitted light; consequently, when seen in bulk, it 
appears of a less pure white. Why should not magnesia be 
used as a pigment? A painter will tell you it has not sufficient 
“body ;”” and now comes the question, What is “ body,” and 
why has one white powder more than another? Before we 
can give an intelligent answer to this question, we must take 
into account the laws which govern the transmission, absorp- 
tion, and reflection of light. 
_ Light falling obliquely upon a plate of glass is partly re- 
flected, partly transmitted, until it comes to the under sur- 
face of the glass, where it is asecond time partly reflected and 
partly transmitted; the reflected portion again meeting the 
upper surface, is subjected to further division into reflected 
and transmitted light ; the reflected portion meets the lower 
surface, is turned up again, and so ad infinitum. 
In the diagram we have roughly sketched the path of a 
ray. 
A and B are sections of glass plates : 1, is the incident ray ; 
2, 3, 4, 5, &e., indicate the reflections it undergoes within the 
substance of the glass, the intensity of the beam becoming 
