26 



On some Optical Phenomenon, ly Captam McBaJciti. 



Oue of tlie most beautiful effects of Nature is that presented 

 at sunset, ^^•llen not only mountain peaks and cliffs glow in the 

 ruddy light, but the most flat and at other times iminteresting 

 country, puts on the appearance of an immeasurable expanse co- 

 vered with a luminous and gloriously tinted haze. In foggy smoky 

 grimed London, looking westward from the Marble Arch, sun- 

 sets may sometimes be seen of unsurpassed beauty. It seems as 

 if out of the very imperfection of things, smoke and damp and 

 fogs, beauty grew as flowers do out of the unsightly soil. As 

 the rainbow is the token of the covenant of promise, so the sun- 

 set has been looked iipon as a memorial of the time •when God 

 ■walked with Adam in the cool of the evening. Doubtlessly these 

 things have their moral as ■well as their physical significance. It 

 ■would be as unscientific to ignore the one as the other. The 

 question is often asked, -what is the cause of the predominance 

 of the red raj's at sunset ? It is sometimes stated that it is due 

 to the red rays being stronger than the rest, so that they pene- 

 trate the clouds -when the others, so to speak, are filtered out 

 or absorbed. If this were the case ■we ought to have the same 

 effects, to a less extent, at mid-day. Or it is ascribed to the 

 polarisation of light, which being a very good sounding term, 

 generally proves sufficient for, if it does not satisfy, the ques- 

 tioner. But is it not rather mainly due to the atmosphere 

 rapidly decreasing in density having the same optical effect as a 

 triangle glass prism, ■which to reverse the case ■would bring 

 about similar results of refraction if it ■were possible to make a 

 rectangular one gradually decreasing in density upwards ? The 

 ■white light being in this manner analysed into its seven pri- 

 mar}' colours, absorption at the same time taking place depend- 

 ing upon varj'ing conditions of the atmosphere, red, golden, or 

 silver sunsets are produced. The minglings of the red rays ■with, 

 and their diffusion through the clear blue sky at sunrise, very 

 rarely in this climate produces the beautiful red purple sky, 

 which I ■witnessed one June morning, about sixteen 

 years ago, at Hj-the. A curious and striking optical 

 phenomenon may be seen in Canterbui\v Cathedral of an after- 

 noon when the gas is lighted in the choir. If a position be 

 taken close to the west end of the south aisle of the choir, and 

 tlie ■wood carving of the organ screen be looked at through the 

 glass, filling up the spaces above the stalls, it will appear as if 

 painted a chrome yellow, in some places almost ec[ual to a dead 

 gold surface, ■with a purple blue fillet in the architrave and silver 

 ornaments, the effect being very conspicuous, bearing a resem- 

 blance to Majolica ware, in brilliancy and contrast. The coloui-- 

 ing is so distinct that it is almost impossible to believe that it 



