produced in the Prismatic Spectrum. 85 



an extent as altogether to arrest the passage of the light, and 

 the space from F to G, which in fig. 4- represents several well- 

 marked groups, becomes one continuous band of shadow. 

 On allowing the tube to cool, the lines in this part resume 

 their former distinctness. Analogous phaenomena are ob- 

 served in the case of iodine; this vapour acts principally on 

 the green portions of the spectrum, and if the vapour be at all 

 dense, the orange and yellow spaces appear crossed by the 

 equidistant lines (Plate II. fig. 2) which characterise this 

 vapour; and they gradually are lost in the green, which seems 

 blotted out by a uniform shadow. As the tube is allowed to 

 cool, and the mass of vapour becomes rarer and its hue fainter 

 and fainter, the lines are seen by degrees extending into the 

 green, and at length the whole shaded portion is filled with 

 them : they do not disappear in this part until those in the 

 orange and yellow are gone, and until the last tinge of violet 

 in the tube has completely passed away. In most instances, 

 where vapours, and not gases, are the subjects of experiment, 

 it will be found advantageous to exhaust the air and seal the 

 tube by the flame of a blowpipe. The vapour of indigo and 

 of several other bodies is thus experimented on without diffi- 

 culty. 



I need not advert to the remarkable but now well-ascer- 

 tained fact, tha^ in the light of the afternoon and evening, lines 

 become visible which at other times are not readily detected, 

 and that those of the red and orange portions of the spectrum, 

 in particular, become much more distinctly marked; but a 

 singular appearance accidentally presented itself to me the 

 other day. I was examining the spectrum of the diffused day- 

 light towards the evening when a violent thunder-shower came 

 on; lines not before visible were distinctly apparent, and a 

 group in the brightest part of the spectrum between D and E, 

 though nearer to the former line, became very evident, in- 

 creasing in distinctness with the violence of the shower ; as the 

 rain passed away they again faded and disappeared. I have 

 had subsequent opportunities on several occasions of confirm- 

 ing the accuracy of this observation. 



In fig. 1, I have represented approximatively some of the 

 principal groups of lines that I have observed in daylight after 

 transmission through different vapours. No pretensions to 

 accuracy are made in these sketches; they are simply in- 

 tended to convey an idea of the general position and grouping 

 of the lines. 



The two first series, those of bromine and iodine, are pro- 

 duced by simple bodies; the remaining ones by compounds 

 containing only two elements, it is true, but the arrangement 



