346 HOW TO OBSERVE THE HEAVENS. 



the earth, that is, than a globe whose bulk would be above ten million times 

 greater than that of the sun. The telescope showed, however, that the ap- 

 pearance of magnitude was altogether illusory and dependent, on atmospheric 

 phenomena ; for, though upon hazy or troubled nights stars may appear 

 large, their magnitude is not permanent, but accompanied with a boiling, 

 tremulous, or bubbling outline. And in good climates and still nights no 

 micrometer will give a sensible outline or apparent diameter to any but large 

 stars. That such, when viewed through large and powerful telescopes, may 

 exhibit some slight sensible magnitude, may be true, but it is demonstrable by 

 the admitted principles of optics, that even a lucid point, if such could exist, 

 could never appear as a mere point through any telescope constructed with 

 spherical refracting or reflecting surfaces. The term magnitude, therefore, 

 must be understood as expressing merely apparent brightness, which, not 

 being capable of being exactly measured as to degree, must have an indefinite 

 application.* 



The stars which have been placed in the first class of magnitude amount to 

 about twenty in number, and these differ from each other considerably in ap- 

 parent splendor. Let any one look at the Dog-star, and then immediately turn 

 his eye to Ursa Majoris, or to d Orionis, and he will be immediately con- 

 scious of this. If the brightness of the latter be expressed by 100, Sir John 

 Herschel estimates that of the Dog-star at 324. 



If the brightness of a star of the sixth magnitude (the smallest distinctly 

 visible to ordinary eyes without a telescope), be supposed to be expressed by 

 1, the brightness of those of superior magnitudes will, according to Sir William 

 Herschel, be expressed as follows : 



Brightness of a star of the average 6th magnitude 1 



Ditto ditto oth 2 



Ditto ditto 4th 6 



Ditto ditto 3d 12(7) 



Ditto ditto 2d 25 



Ditto ditto 1st 100 



Of these estimates, that astronomer considered the third as doubtful, the others 

 more exact. 



We have already observed that the telescope augments our range of vision 

 by rendering perceptible stars which are lost to the eye by reason of their dis- 

 tance. It also multiplies the objects visible to us, even in the radius which cir- 

 cumscribes stars of the sixili magnitude. It may however be asked, how it 

 is that the telescope can effect this, seeing that it is incapable of presenting 

 any star, even the largest, to the eye, as anything but a lucid point, without 

 definite outline or appreciable magnitude. Let us therefore explain this. 



In the front of the eyeball is a colored annular membrane called the iris, 

 in the centre of which appears a circular black spot. This spot is not a black 

 substance, but is an aperture which seems black only because the chamber 

 within it is dark. This aperture is the window of the eye provided for the 

 admission of light. On the inside of the eye, lining the inner surface opposite 

 this window, is the membrane called the retina, endowed with a specific sen- 

 sibility, in virtue of which, when light strikes upon it, an effect is conveyed 

 by the nerves therewith connected to the seat of sensation, by which vision is 

 effected. 



The sensibility of the retina is limited. Light may act on it so slightly as 

 to produce no perception. To produce vision, therefore, it is not enough that 

 light be admitted through the pupil ; it must enter in sufficient quantity. Let 

 us then suppose the eye directed toward a luminous object such as a star 



* See De Morgan on the Maps of the Stars, p. 79. 



