36 INDEX CATALOGUE. 



Herschel, W. : Abstracts of Memoirs — Continued. 



A.D. Vol. P. 



Star gauging gave rise to an investigation of the spaec-pen- 

 etrating power of telescopes. 

 1317 107 310 Finding that this might be calculated with reference to 

 the extent of the same power of which the unassisted 

 eye was capable, there always remained a desideratum of 

 some sure method by which this last might be ascertained. 

 Description of experimental apparatus. 

 311 Method of limiting apertures. 

 313 VI. Of the extent of natural vision. 

 313-8 Experiments on stars. 



318 The distances of. clusters cannot be ascertained by the 



method of equalizing star-light. 

 VII. Of the extent of telescopic vision. 



319 Experiments which go to show that the diameter of the 



pupil of the human eye is not more than 0.21 inch, and 

 greater than 0.17 when observing with a telescope. It 

 may be assumed 0.2 inch. 



320 VIII. Application of the extent of natural and telescopic 



vision to the probable arrangement of the celestial bodies 

 in space. 

 We shall be able to say that a distant celestial object is so 

 far from us, provided the stars of which it is composed 

 are of a size and lustre equal to the size and lustre of 

 such stars as Sirius, Arcturus, etc. 



321 The stars of the tenth, eleventh, and twelfth order of dis- 



tances are not only more compressed than those in the 

 neighborhood of the sun, but, moreover, their compres- 

 sion in different parts of the heavens must be very un- 

 equal. 

 IX. Of the construction and extent of the milky way. 



322 General description of it. 



The sun is within its plane, for to an observer in latitude 

 60°, when at 100° 11. A. the milky way is in the east, it 

 will at the same time be in the west at 280°; while in it- 

 meridional situation it will pass through Cassiopeia in the 

 Zenith and through the constellation of the Cross in the 

 Nadir. 

 323-4 Examination of the cluster in the Sword Handle of Per- 

 seus, with various space-penetrating powers. 



325 [Beside the 863 gauges published in P. T. , 178-3, p. 221, 



above 400 more have been taken in various parts of the 

 heavens.] 



326 The twenty-foot telescope cannot fathom the profundity ;>f 



the milkv wav. 



