152 Of Optics.

Any of these colours, except red and violet, may be made by mixingtogether the two contiguous prismatie colours. Thus, yellow is madeby mixing together a due proportion of orange and green j and greenmay be made by a mixture of yellow and blue.

All bodies appear of that colour, whofe rays they reflect most; as abody appears red when it reflects most of the red-making rays, andabforbs the rest.

Transparent Any two or more colours that are cluite transparent by themselves,colours be- become opake when put together. Thus, if water or fpirits of wine beput together. tinged red, and put in a pbial, every object feen through it will appearred ; becaufe it lets only the red rays pafs through it, and stops ali therest. If water or fpirits be tinged blue, and put in a phial, all objectsfeen through it will appear blue, becaufe it tranfmits only the blue rays,and stops ali the rest. But if these two phials are held clofe together,so as both of them may be between the eye and obsect, the object willno more be feen through them than through a piate of metal; forwhatever rays are tranfmitted through the fluid in the phial next theobject, are stopped by that in the pbial next the eye. In this experi-ment, the phials ought not to be round, but fquare; becaufe nothingbut the light itself can be feen through a round transparent body, at anydistance.

As the rays of light suffer different degrees of refraction by pastingobliquely through a prifm, or through a convex glafs, and are therebyseparated into all the feven original or primary colours ; so they alsosuffer different degrees of refraction by pasting through drops of fallingrain; and then, being reflected towards the eye, from the sides of thesedrops which are farthest from the eye, and again refracted by pastingout. of these drops into the air, in which refracted directions theycome to the eye; they make all the colours to appear in the form ofa fine arch in the heavens, which is called the rain-bow.

There are always two rain-bows feen together, the interior ofwhich is formed by the rays a b y which falling upon the upper partl-ig. 7. b, of the drop bcd, are refracted into the line ^ c as they enter thedrop, and are reflected from the back of it at c, in the line c d, and then,by pasting out of the drop into air, they are again refracted at d ; andfrom thence they pafs on to the eye at e: so that to form the interiorbow, the rays suffer two refractions, as at b and d ; and one reflection,as at c .

The exterior bow is formed by rays which suffer two reflections,and two refractions; which is the occasion of its being lefs vivid thanthe interior, and also of its colours being inverted with refpect to thofe. - s of