212 HERSCHEL. 



Under equality of subtended angle, then, the telescopic vision witli 

 strong magnifying powers showed itself superior to the naked-eye vision. 

 This result is not unimportant. 



If we take notice of the magnifying powers used by Ilerschel in these 

 laborious researches — powers that often exceeded live hundred times — it 

 will appear to be established that the telescopes possessed by modern 

 astronomers may serve to verify the round form of distant objects, the 

 form of celestial bodies, even when the diameters of those bodies do not 

 subtend naturally, to the naked eye, angles of above three- tenths of a 

 second; and 500 multiplied by three-tenths of a second give 2' 30". 



Much still remained to be learned in regard to refracting telescopes ; 

 even when they already served to reveal brilliant astronomical phenom- 

 ena; the result was due rather to chance than to detinite theory. Their 

 theory, as far as it depended on geometry and optics, had made rapid 

 ]3rogress. These two early phases of the problem leave but little more 

 to be wished for; it is not so with a third phase, hitherto a good deal ne- 

 glected, connected with physiology, and with the action of light on the 

 nervous system. Therefore, we should search in vain in old treatises on 

 optics and on astronomy for a strict and complete discussion on the 

 comparative effect that the size and intensity of the images that the 

 magnifying power and the aperture of a telescope may have, by night 

 and by day, on the visibility of the faintest stars. This lacuna Herschel 

 tried to fill up in 1799; such was the aim of the memoir entitled "On 

 the Space-penetrating Power of Telescopes." 



This memoir contains excellent things; still it is far from exhausting 

 the subject. The author, for instance, entirely overlooks the observa- 

 tions made by day. I also find that the hypothetical part of the dis- 

 cussion is not perhaps as distinctly separated from the rigorous part as 

 it might be; that doubtful numbers, though given with a degree of 

 precision down to the smallest decimals, do not contrast well as terms 

 of comparison with those which, on the contrary, rest on observations 

 bearing mathematical evidence of correctness. 



Whatever may be thought of these remarks, the astronomer or the 

 physicist who would like again to undertake the question of visibility 

 with telescopes will find some important facts in Herschel's memoir, 

 and some ingenious observations, well adapted to serve them as guides. 



LABORS IN SIDEREAL ASTRONOMY. 



The curious phenomenon of a i)eriodical (;hange of intensity in certain 

 stars very early excited the earnest attention of Herschel. The first me- 

 moir by that illustrious observer, presented to the Eoyal Society of Lon- 

 don, and inserted in the Philosophical Transactions, treats especially 

 of the changes of intensity of the star o in the neck of the Whale. 



This memoir was dated from Biith, May, 1780. Eleven years afterward, 

 in the month of December, 1791, Herschel communicated a second time 

 to that celebrated English society the observations that he had jiiade by 



