564 BIBIJOGRAPHY Ob' HERSCHEL'S WKITINGfS. 



Herschel, W,: Synopsis of the AVuitings of— Contiuued. 

 A. D. Vol. P. 



1800 90 51 The greatest iuequalities iu the brightness of luminous bodies will be 

 owing to their natural texture. . 

 51 Brightness, I ascribe to bodies that throw out light, and those that 

 throw out most are the brightest. 



51 Let the whole quantity of light thrown out by a luminous body be, L ; 



suppose the surface of this body to be composed of N luminous phys- 

 ical points. Let C stand for the mean copiousness of light thrown 

 •out from all the physical points of a luminous object. Let c express 

 the copiousness of emitting light of any number of physical iioints 

 that agree in this respect, and let their number be n ; similarly I 

 define c' and n^ for another set of j)oints, and so on. Then L = en 

 4- cijji -{- d^n" + etc., and L -:- N = C. 



52 An objection to this answered. 



5'2 The brightness of an object is truly defined by CN. The brightness 

 arising from the great value of C may be called the intrinsic bright- 

 ness ; that from the great value of N the aggregate brightness ] the 

 absolute brightness is in all cases CN. 



53 In finding an expression for the apjiearance of luminous objects at any 



assigned distance we must leave out of account every part of CN 

 which is not applied for the purposes of vision. L representing the 

 whole light thrown out by CN, let I be that part of it used in vision 

 either by the eye or the telescope. 



53 The equation of light, in this sense, is CN = /. 



53 The expression for its quantity at the distance of the observer D will 

 be I H- D". 



53 In natural vision I undergoes a considerable change by the opening 

 or contracting of the pupil of the eye. 



53 In some experiments on light made at Bath iu 1780 I [noticed the in- 

 creased power of vision after staying some time in a dark room]. 



53 The opening of the iris is probably not the only cause of seeing better 



after remaining long in the dark, but the tranquillity of the retina 

 may render it more fit to receive impressions. 



54 This is supported by telescopic vision, for in my sweeps of 4, 5, or 6 



hours' duration my eye became so sensitive that when a star of the 

 3d magnitude came towards the field of view I had to withdraw my 

 eye in order not to injure the delicacy of vision. The opening of 

 the iris was not the cause, as the diameter of the 'optic pencil was 

 no more than 0.12 inch. 



54 With the 40-foot telescope the appearance of Sirius announced itself 



like the dawn of the morning till this brilliant star at last entered 

 the field of the telescope with all the splendor of the rising sun, and 

 forced me to take my eye from that beautiful sight. 



55 I found the eye, coming from the light, required nearly 20 minutes 



before it could see delicate objects, and that the view of a star of 

 the 2d or 3d magnitude would so disorder it that nearly the same 

 time was required to reestablish its tranquillity. 



55 If a is the opening of the iris and A the aperture of the telescope -^^ 



A^L 



is the light admitted by the eye, and -t^t will be that admitted by 



the telescope. 

 55 Whenever the pencil of light from the telescope is larger than the 

 aperture of the pupil, light is lost. If m be the magnifying power, 

 A-^m ought not to exceed a. 



