444 



4.stronomical and Nautical Collections. 



white stripe, the light becomes gradually coloured ; the co- 

 lours of the second fringe are very distinct, as well as those 

 of the third and fourth ; but they afterwards become fainter ; 

 and they totally disappear after the eighth fringe, by the 

 more complete mixture of the dark and bright stripes of all 

 the colours, which produces a uniform white light. 



If we make the experiment, which has been described, 

 with the rays of the seven principal colours which Newton 

 distinguishes in the solar spectrum, and measure the breadth 

 of the stripes with the assistance of the micrometer already 

 mentioned, it will of course be possible to deduce from it, 

 by calculation, the corresponding values of d; but this 

 observation has only been made with care on the red light, 

 tolerably homogeneous, that is transmitted by certain 

 coloured glasses used for the windows of churches. For the 

 rays which preponderate in this light, and which belong to 

 a part very near the end of the spectrum, the length of d 

 is . 00002512 inch, English. We may derive the value of dy 

 for the seven principal species of rays, from the observations 

 of Newton on coloured rings ; it is sufficient for this pur- 

 pose, as will hereafter appear, to multiply by 4 the lengths 

 of what he calls fits of easy reflection, or of easy transmis- 

 sion of the luminous molecules. In this manner the follow- 

 ing table has been calculated : — 



Limits of the pein- 

 ciPAL Colours. 



Extreme violet 

 Violet indigo . 

 iDdigo blue 

 Blue green 

 Green yellow . 

 Yellow orange . 

 Orange red . . 

 Extreme red . 



Extreme values 



of d. 

 C .0000160 i 

 ■[[.0000167 Y] 

 (■ .0000173 

 t[. 0000181] 

 f .0000181 

 t[. 0000189] 

 (■ 0000194 

 t[. 0000203] 

 f .0000210 

 t[. 00002 19] 

 f .0000218 

 t[. 0000235] 

 j" .0000235 

 U- 0000246] 

 J .0000254 

 U. 0000266] 



Principal 

 Colours. 



Violet 



Indigo 



Blue . . 



Green 



Yellow 



Orange 



Red . 



Intermediate 

 values of d. 



f .0000167 i 

 U. 0000.174 Y] 

 f .0000177 

 "[[.0000185] 

 f .0000187 

 "[[.0000196] 

 f .0000202 

 ■[[.0000211] 

 / .0000217 

 "[[.0000227] 

 f .0000230 

 ■[[.0000240] 

 5 .0000244 

 ^ [.0000256] 



[The numbers witliin brackets are those which Dr. Young inserts in his 

 table as deiived from Newton's experiments.] 



