231 



SCIENTIFIC SIDE-LIGHTS 



Exactness 



bubble, and he calculated the corresponding 

 thickness. How he did this may be thus 

 made plain to you: Suppose the water of 

 the ocean to be absolutely smooth ; it would 

 then accurately represent the earth's curved 

 surface. Let a perfectly horizontal plane 

 touch the surface at any point. Knowing 

 the earth's diameter, any engineer or mathe- 

 matician in this room could tell you how far 

 the sea's surface will lie below this plane, at 

 the distance of a yard, ten yards, a hundred 

 yards, or a thousand yards from the point 

 of contact of the plane and the sea. It is 

 common, indeed, in leveling operations, to 

 allow for the curvature of the earth. New- 

 ton's calculation was precisely similar. His 

 plane glass was a tangent to his curved one. 

 From its refractive index and focal distance 

 he determined the diameter of the sphere of 

 which his curved glass formed a segment, he 

 measured the distances of his rings from the 

 place of contact, and he calculated the depth 

 between the tangent plane and the curved 

 surface exactly as the engineer would calcu- 

 late the distance between his tangent plane 

 and the surface of the sea. The wonder is 

 that, where such infinitesimal distances are 

 involved, Newton, with the means at his dis- 

 posal, could have worked with such marvel- 

 ous exactitude. TYNDALL Lectures on 

 Light, lect. 2, p. 74. (A., 1898.) 



1124. 



Pasteur's Care in 



Experiments No Life in Glacier Air. The 

 caution exercised by Pasteur, both in the 

 execution of his experiments and in the rea- 

 soning based upon them, is perfectly evident 

 to those who, through the practise of severe 

 experimental inquiry, have rendered them- 

 selves competent to judge of good experi- 

 mental work. He found germs in the mer- 

 cury used to isolate his air. He was never 

 sure that they did not cling to the instru- 

 ments he employed, or to his own person. 

 Thus when he opened his hermetically sealed 

 flasks upon the Mer de Glace, he had his eye 

 upon the file used to detach the drawn-out 

 necks of his bottles, and he was careful to 

 stand to leeward when each flask was 

 opened. Using these precautions, he found 

 the glacier air incompetent, in nineteen 

 cases out of twenty, to generate life; while 

 similar flasks, opened amid the vegetation of 

 the lowlands, were soon crowded with living 

 things. TYNDALL Floating Matter of the 

 Air, p. 33. (A., 1895.) 



1125. 



Precise Quantita- 



tive Measurements Needed Refraction of 

 Light Kepler a Theorist on the Observa- 

 tions of Others The " Personal Equation " 

 in Science. As regards the refraction of 

 light, the course of real inquiry was re- 

 sumed in 1100 by an Arabian philosopher 

 named Alhazen. Then it was taken up in 

 succession by Roger Bacon, Vitellio, and 

 Kepler. One of the most important occu- 

 pations of science is the determination, by 

 precise measurements, of the quantitative 

 relations of phenomena; the value of such 



measurements depending greatly upon the 

 skill and conscientiousness of the man who 

 makes them. Vitellio appears to have been 

 both skilful and conscientious, while Kep- 

 ler's habit was to rummage through the ob- 

 servations of his predecessors, to look at 

 them in all lights, and thus distil from them 

 the principles which united them. He had 

 done this with the astronomical measure- 

 ments of Tycho Brahe, and had extracted 

 from them the celebrated " laws of Kepler." 

 He did it also with Vitellio's measurements 

 of refraction. But in this case he was not 

 successful. The principle, tho a simple one, 

 escaped him, and it was first discovered by 

 Willebrod Snell, about the year 1621. 

 TYNDALL Lectures on Light, lect. 1, p. 14. 

 (A., 1898.) 



1126. Specimens Once 



Vaguely Located Loose Methods Now Dis- 

 carded. Fifty years ago the exact locality 

 from which any animal came seemed an un- 

 important fact in its scientific history, for 

 the bearing of this question on that of origin 

 was not then perceived. To say that any 

 specimen came from South America was 

 quite enough; to specify that it came from 

 Brazil, from the Amazons, the San Fran- 

 cisco, or the La Plata, seemed a marvelous 

 accuracy in the observers. In the museum 

 at Paris, for instance, there are many speci- 

 mens entered as coming from New York or 

 from Para ; but all that is absolutely known 

 about them is that they were shipped from 

 those seaports. Nobody knows exactly where 

 they were collected. So there are specimens 

 entered as coming from the Rio San Fran- 

 cisco, but it is by no means sure that they 

 came exclusively from that water-basin. All 

 this kind of investigation is far too loose for 

 our present [1865] object. AGASSIZ Jour- 

 ney in Brazil, ch. 1, p. 9. (H. M. & Co., 

 1896.) 



1127. 



Velocity of Light 



Determined. The velocity of light, as is 

 well known, was first determined by irregu- 

 larities in the time of the eclipses of Jupi- 

 ter's satellites, which were found to occur 

 earlier or later than the calculated times, 

 according as we were near to or far from 

 the planet. It was thus found that it re- 

 quired [about] eight minutes for light to 

 travel from the sun to the earth, a distance 

 of a little more than ninety millions of 

 miles; so that light travels about 186,000 

 miles in a single second of time. It would 

 seem at first sight impossible to measure the 

 time taken by light in traveling a mile, yet 

 means have been discovered to do this, and 

 even to measure the time taken for light to 

 traverse a few feet from one side of a room 

 to the other. Yet more, this method of 

 measuring the velocity of light has, by suc- 

 cessive refinements, become so accurate that 

 it is now considered to be the most satisfac- 

 tory method of determining the mean dis- 

 tance of the sun from the earth, a distance 



