68 



OPTICS. 



these phenomena 'can be determined. 

 The principal facts which have been 

 ascertained, we* owe to the ingenuity of 

 M. Bouguer and M. Lambert, and these 

 vre shall now lay before the reader : 



Number of Rays reflected oat of 1000. 



Ancteof Water. Gla. Quick- Silver. Platter. Dutch 

 J.. EM* Uver. Paper. 



894 721 .. 721 .. .. ... 



89 692 



884 669 



88 639 



874 614 584 



85 501 543 



82J 409 474 



80 333 412 



77J 271 356 



75 211 299 .. 209 194 203 



724 178 222 



70 145 210 



65 97 156 



60 65 112 .. 319 352 332 



50 34 57 



45 41 .. 455 529 507 



40 22 34 704 



30 19 27 .. 640 640 .. 



20 18 25 



15 18 25 .. 802 762 971 



10 18 25 



18 25 6C6 1000 1000 1000 



From these results we may draw the 

 following conclusions : 



1. That m fluids, transparent solids* 

 and mctalx, the quantity of light 

 reflected increases with the angle 

 of incidence reckoned from the 

 perpendicular; whereas in \\hite 

 opaque bodies the quantity of liiiht 

 reflected decreases with the angle 

 of incidence. 



2. That at great angles of incidence 

 water reflects more liuht than even 



!i the suri. 



The following very accurate results 

 were obtained by M. Lambert, \\Iio 

 measured the quantity of light reflected 

 both at the first and the second surfaces; 



carefully comparing them, we have 

 found that the quantity of light re- 

 flected at either surface is inversely as 

 the square of the cosine of the angles 

 of incidence. It is a most curious fact 

 which Lambert has established, that 

 the light reflected at the second surface 

 of a plate of glass is at all angles of 

 incidence more than double of the quan- 

 tity reflected by the first surface. 



M. Lambert likewise obtained the 

 following results for different numbers 

 of very transparent plates of glass at a 

 perpendicular incidence. 



70 



60 



50 



40 



30 



'20 



in 

 



!.ot seem to have ob- 

 icsults. Upon 



No. of Plates 



rfGlM. 



Xo. of 



Ray reflected 

 out of 1000. 



62 

 117 

 165 

 209 

 248 

 284 

 316 

 345 



Light 

 transmitted. 



938 



883 

 835 

 791 

 752 

 715 

 C84 

 654 



When the glass which he used was 

 less transparent, he obtained the follow- 

 ing results, the quantity of light lost 

 irivcn in the last column." 



1 



2 



I 



4 



. 8 

 10 

 32 



No. of It ayK 

 . 

 of looo. 



52 



80 

 108 

 123 

 J 17 

 152 

 153 



Xo. of Hay < 

 transmitted 

 out of 1000. 



811 

 600 

 5*7 



438 



1 95 



89 



1 



Xo. of JJa.tf 



lust out of 



1000. 



137 



355 

 449 

 659 



809 

 846 



From the preced i 



n;g fact s it is obvious, 



that in all the \aiious (,];eratioi.- 

 Which we distribute or concentrate linht 

 for economical JIUIJIOM-S, a consider- 

 able portion of it is lost. This por- 

 tion is much greater in metals Ilian 

 in glass ; and hence, v hen other c.r- 



!:mces ai-e the same, /r//.sv.s 

 ])i'ctci-ablc to wirrorx or .\j/rcu/u, i-ilher 



For concentrating the solar ia\s for the 



JJUIJMJSCS of (-(.mljiistion, or lor j in- 

 ducing in li-hl-lnnivcs an intensely 

 brilliant column of li^ht caj:ab:- 



iiini; to a irreat distance, and jiene- 



trating the fogf of the ocean. Forilu! 



same n-aMin a Hi-fnirliiiir tele cnj : 

 gi\es far more li-ht than a 7V//f'7///- 

 of the same aj eitiiie ; and if \\e 

 could manufacture uhiss as easil\ as \\ ( 

 CUM c;i^l metallic sj, ecula, the Keilceling 

 1clcM-oj;c \\ould dLsa | | ( ;;i- fioiii among 

 optical instruments in actual use. 



