PHO 



PHR 



light, as in a cloudy or bright day, or be- 

 tween bodies illuminated in different de- 

 grees. The raiio of the intensities uf two 

 luminous objects has been attempted 10 be 

 measured, by placing them at different dis- 

 tances from a given object, until tiiatobject 

 cast two shadows ot equal darkness ; or by 

 observing when two equal objects appear- 

 ed to be equally illuminated each by one of 

 the luminous objects ; for then, by a well 

 known and established law, the proportion 

 of the intensities of their light was suppo- 

 sed to be as the squares of the distances. 

 Thus if two equal objects appear to be 

 equally illuminated, w hen one of them is 

 three feet from a tallow candle, and when 

 the other is nine feet from a wax candle, 

 then it is inferred that the intensity of the 

 light of the former candle is to that of the 

 latter as nine to eighty-one. Mr. Leslie has 

 more recently invented an instrument of 

 this kind, the essential part of which con- 

 sists of a glass tube like a reversed sy- 

 phon, whose two branches should be 

 equal in height, and terminated by balls 

 of equal diameter ; one of the balls is of 

 black enamel, and the other of common 

 glass, into which is put some liquid. 



The motions of the liquor, which is sul- 

 phuric acid tinged red with carmine, are 

 measured by means of a graduation, the 

 zero is situated towards the top of the 

 branch that is terminated by the enamel- 

 led ball. The use of this instrument is 

 founded upon the principle, that when 

 the light is absorbed by a body, it pro- 

 duces a heat proportional to the quantity 

 of absorption. When the instrument is 

 exposed to the solar rays, those rays that 

 are absorbed by the dark colour, heat the 

 interior air, which causes the liquor to 

 descend at first with rapidity in the cor- 

 responding branch. But as a part of the 

 heat, which had introduced itself by means 

 of the absorption, is dissipated by the ra- 

 diation, and as the difference between the 

 quantity of heat lost and that of the heat 

 acquired goes on diminishing, there will 

 be a point where, these two quantities hav- 

 ing become equal, the instrument will be 

 stationary, and the intensity of the incident 

 light is then estimated by the number of 

 degrees which the liquor has run over. 

 The author of this ingenious instrument, 

 has pointed out its advantages in deter- 

 mining the progressive augmentation un- 

 dergone by the intensity of the light, and 

 the gradation in a contrary sense, which 

 succeeds to that progress, both from the 

 beginning of day to its end, and from the 

 winter solstice to the end of the succeed- 

 ing autumn. With the help of such an 



instrument, one might also compare the 

 action of rays of light in different coun- 

 tries, of winch some dart with sufficient 

 constancy from a fine and serene sky, 

 while others seem to be covered with a 

 vtil which dims and obscures their lustre. 

 Mr. Leslie, having proposed to himself to 

 measure the energy of the several colour- 

 ed rays which compose the solar spec- 

 trum, caused a beam of light to pass 

 through a prism of flint glass; and the 

 indications of the photometer presented 

 successively to the different parts of the 

 spectrum, have furnished nearly for the 

 relation, between the degrees of force of 

 the blue, green, yellow, and red rays, that 

 of the numbers 1, 4, 9, 16 ; a relation 

 which, considered in the two extreme 

 terms, is more than quadruple that which 

 was substituted for it by Dr Her&chel, 

 who has made experiments for the same 

 purpose. 



PHRYGANEA ,in natural history, a ge- 

 nus of insects of the order Neuroptera. 

 Generic character: mouth with a horny 

 short curved mandible ; feelers four ; 

 three stemmata ; antennae setaceous, long- 

 er than the thorax ; wings equal, incum- 

 bent, the lower ones folded There are 

 nearly sixty species, in two divisions. A. 

 Tail with^ two truncate bristles. B. Tail 

 without Bristles. The insects of this ge- 

 nus are seen in a summer's evening float- 

 ing in the air in large masses, and are ea- 

 gerly devoured by swallows. They re- 

 semble moths, particularly the division 

 called Tineae ; but may readily be distin- 

 guished by their feelers, and also by the 

 stemmata situated at the top of the head. 

 The phryganeae proceed from aquatic lar- 

 vae of a lengthened shape, residing in tu- 

 bular cases, which they form by aggluti- 

 nating various fragments of vegetable sub- 

 stances, &c. These tubular cases are 

 lined within by a tissue of silken fibres, 

 and are open at each extremhy. The in- 

 cluded larvae, when feeding, protrude the 

 head and fore-parts of the body, creeping 

 along the bottom of the waters they in- 

 habit, by means of six short and slender 

 legs ; on the upper part of the back is a 

 sort of prop, preventing the case, or tube, 

 from slipping too far forwards during the 

 time the animal is feeding. One of the 

 largest species is the P. grandis, (see 

 Plate IV. Entomology, fig. 2.) This in- 

 sect is about an inch in length, very like 

 a phaljena ; the upper wings are grey, 

 marked by various darker and lighter 

 streaks and specks, and the under wings 

 yellowish brown, and semitransparent. 

 The larvae of this genus is known by the 



