Census op the sky — Sampson. 187 



infer the continuation ; and, if these grades are incorrectly or debat- 

 ably demarcated from one another, your results are liable to such 

 enormous uncertainties that they can hardly be held to add anything 

 to knowledge. To have performed this counting, as I believe it 

 has been effectively and securely performed, is, in my judgment, 

 a vei-y great feat, one that would appropriately be taken as a land- 

 mark in the history of the mind; and I do not think I detract 

 from this at all if 1 say that those who have actually done the work 

 would not lay claim to more than to have well and truly performed 

 a straightforward task by established methods. None the less, it 

 marks a stage, a fact among many surmises, an achievement among 

 many attempts. 



Counting the stars is nothing else than the method of Herschel's 

 star gauges supplemented by a due consideration of all the diffi- 

 culties which he overstepped by intrepid assumptions. AYlien Her- 

 schel set up his 20-foot reflector of 18-inch aperture it was mounted 

 vertically in the meridian with a sweep of a little more than 2^°, 

 and he surveyed the sl^ in zones of declination, taking everything 

 that came by, and, in particular, counting the density of the fields. 

 These counts were the bases of his papers on the " Construction of 

 the heavens," which showed that the sun was roughly in the center 

 of an irregular disk-shaped universe of stars, researches that I have 

 heard Sir David Gill describe as " almost inspired." But, if he was 

 inspired, like other prophetic writers, we have to repose upon his 

 genius, for criticism spoils him. It will not do now to tell us that 

 a seventh-magnitude star may be generally taken as seven times as 

 distant as a first-magnitude star. In the first place, calculation is 

 astray — 25 times would be more defensible — ^but, in the second, 

 though we know that distance must raise magnitude, generally speak- 

 ing, we are quite unable to verify the connection. But, most of 

 all, though Herschel " looked farther into heaven than any man 

 before him," for this purpose he did not look nearly far enough. 

 His statement that in a field of 15' diameter he counted some TO 

 or 80 stars, with occasional fields very much denser^ would indicate 

 that he reached to the thirteenth or fourteenth magnitude. The 

 fifteenth magnitude more than doubles the fourteenth, the sixteenth 

 nearly doubles the fifteenth, the seventeenth nearly doubles the six- 

 teenth. How does the progression continue? Does it go on forever? 

 Does it go on even as far as we can see ? 



No real advance upon Herschel's gauges could be made without 

 photography, both because the record is permanent and so leaves 

 you time to count and also because the faintness of the stars that 

 you can reach is almost unlimited. 



Let me now leave generalities and give you, as succinctly as pos- 

 sible, some details of the work I am describing. 



