50 Dr W. Scoresby on Prismatic Colours in Lew Drops. 



In cases where the gossamer forms a pretty close and 

 regular tissue, and is evenly stretched and rather broadly 

 spread ; and where the dew is profusely deposited on the 

 different fibres of the web, and illuminated by a perfectly 

 clear morning sun, we have the best conditions for the exa- 

 mination of the optical phenomena. For the purpose of the 

 examination, a pocket telescope is the most convenient in- 

 strument ; but it requires to draw out so far beyond the ordi- 

 nary focus, as to afi^ord a clear view of near objects, even as 

 near as 6 or 8 feet to the eye. A convenient size of telescope 

 is such as what I commonly use, a three-draw glass of one- 

 inch aperture, and 12 or 13 inches focal length, drawing 

 out nearly an inch and a half beyond the focal position for 

 distant objects. 



With the back of the observer towards the sun, and whilst 

 the sun is not yet too high in the sky, certain positions will 

 be observed, in which the dew-covered gossamer presents a 

 surface of reflected light, and when, moving the head, slightly, 

 right or left, a tinge of colour, more or less definite, will pro- 

 bably be seen. If the distance of the tissue from the eye be 

 not less than the shortest range of clear vision in the tele- 

 scope, the colours may be then examined and analyzed by the 

 instrument ; and, by slight changes in the place of the ob- 

 server's head, made to pass through the whole of the pris- 

 matic variety. If the distance be too short for the telescope, 

 by reason of the sun's high position in the heavens, that dis- 

 tance may be increased, after the manner already described, 

 by interposition of a table, chair, or other piece of domestic 

 furniture, betwixt the feet of the observer and the ground. 



Observing in one or other of these methods, I have been 

 quite charmed with the splendour of appearance of the dew- 

 bespangled gossamer. Sometimes, where the breadth of the 

 web was considerable, most of the colours of the spectrum 

 have been observed on the same surface, in regular and beauti- 

 ful series ; and these colours made to shift or vanish, as in 

 some optical experiments with polarizing instruments, with 

 surprising eff'ects in beauty. 



In cases where the spider's lines are not very compact, as 

 in some observations recently made at Gateshaw, near Kelso, 



