401 



SCIENTIFIC SIDE-LIGHTS 



Lightning 

 Limit 



human history. If we are lost in conjec- 

 tures when speculating on the ages required 

 to lift up these formations to the height of 

 several thousand feet above the sea, how 

 much more remote must be the era when the 

 same rocks were gradually formed beneath 

 the waters! LYELL Principles of Geology, 

 bk.i, ch. 13, p. 185. (A., 1854.) 



1956. LIMIT OF HUMAN POWERS 



Measuring Star Distances The Earth's 

 Orbit Too Narrow The Distance of but 

 One Star Fairly Measured. To measure 

 star distances the earth's dimensions are 

 altogether too small. No instrument which 

 man will ever make would show the slightest 

 difference in the direction of any star as 

 seen from opposite sides of the earth. But 

 precisely as the measurer of the moon's dis- 

 tance need not leave his observatory, or have 

 a companion observer working at a distant 

 station, if he prefers to trust to the earth's 

 rotation to sway his station from one side 

 to the other so the astronomer, unable to 

 leave the earth to seek, as he would wish, a 

 station millions of miles away, can never- 

 theless avail himself of the earth's motion 

 of revolution around the sun, which in the 

 course of six months will carry the earth 

 from one side of her path to the opposite 

 side, one hundred and eighty-three millions 

 of miles away. One place and the other 

 (any two opposite points of the earth's 

 orbit) may be regarded as two observing 

 stations at the ends of a base-line of this 

 enormous length, laid down, as it were, to 

 extend astronomical survey from the solar 

 system to the stars. 



It might be thought that this base-line 

 could not but be amply sufficient for the pur- 

 pose in view. But so much vaster are the 

 distances of the stars that until quite re- 

 cent years this base-line proved altogether 

 too short for effective measurements, and 

 even now only one star has had its distance 

 fairly measured, while some nine or ten have 

 had their distances roughly estimated. All 

 the rest which have been tried lie so far be- 

 yond our means of measurement as to show 

 no signs whatever of change of place as the 

 earth circuits around that orbit which to 

 our conceptions seems so enormous in ex- 

 tent. PROCTOR Expanse of Heaven, p. 241. 

 (L. G. & Co., 1897.) 



1957. LIMIT OF "PERPETUAL 

 SNOW "Snow-line Highest under the Equator 

 Southern and Northern Slopes Compared 

 Silent Victory of Sunshine. That tem- 

 perature is a very important factor in de- 

 termining the height of that line [the snow- 

 line] is obvious, and hence it follows that 

 as a rule J -he snow-line is higher the nearer 

 the mountains are to the equator. On the 

 south side of Mont Blanc, in the Alps, the 

 limit of perpetual snow is about 9,000 feet 

 above sea-level, while on the Andes, near the 

 equator, it is situated at about the height 

 of 16,000 feet higher than the highest sum- 

 mit of the Alps. For the same reason the 



snow-line is usually higher on the side of a 

 mountain exposed to the sun than on the 

 side turned away from it. It is, for ex- 

 ample, about 1,000 feet higher on the south 

 than on the north side of the Alps. Cms- 

 HOLM Nature-Studies, p. 34. (Hum., 1888.) 



1958. LIMIT OF SENSITIVENESS IN 

 RETINA Ready Motion Compensates Wide 

 Field of Vision Secured. An optical defect 

 which has long been known- to ophthalmol- 

 ogists the inferiority in the sensitiveness 

 of the retinal surface generally to that of 

 the central spot known as the macula lutea 

 is shown by Professor Helmholtz to be 

 fully compensated by the facility and rapid- 

 ity with which we move the eye, in such a 

 manner as to bring the image of the object, 

 or of any part of the object, which we wish 

 to examine minutely, upon this sensitive 

 spot; whilst the field over which our vision 

 ranges with sufficient distinctness to see our 

 special object in combination with its sur- 

 roundings, is far larger than is attainable in 

 any optical instrument of human contri- 

 vance. CARPENTER Nature and Man, lect. 

 15, p. 423. (A., 1889.) 



1959. LIMIT OF THE POWER OF 

 THE MICROSCOPE Structure that Defies 

 Microscopic Analysis. Have the diamond, 

 the amethyst, and the countless other crys- 

 tals formed in the laboratories of Nature 

 and of man no structure? Assuredly they 

 have; but what can the microscope make of 

 it? Nothing. It cannot be too distinctly 

 borne in mind that between the microscopic 

 limit and the true molecular limit there is 

 room for infinite permutations and combina- 

 tions. . . . This first marshaling of the 

 atoms, on which all subsequent action de- 

 pends, baffles a keener power than that of 

 the microscope. When duly pondered, the 

 complexity of the problem raises the doubt, 

 not of the power of our instrument, for that 

 is nil, but whether we ourselves possess the 

 intellectual elements which will ever enable 

 us to grapple with the ultimate structural 

 energies of Nature. TYNDALL Fragments of 

 Science, vol. ii, ch. 8, p. 125. (A., 1897.) 



1960. LIMIT OF VISUAL FIELD 

 CONCENTRATES ATTENTION Practical 

 Perfection of the Eye. Now while the dis- 

 advantage of the limitation of distinct 

 vision to the macula lutea [the sensitive 

 spot of the retina] is thus fully compen- 

 sated, I hold that this limitation is posi- 

 tively advantageous in this way that we 

 see the object, or the part of the object, at 

 which we will to look with much greater 

 distinctness than we should do if the whole 

 of the visual picture which we receive at 

 one time were as complete and vivid as that 

 portion of it which is formed on the central 

 spot of the retina. For our mental recep- 

 tivity of this picture depends upon the at- 

 tention we give it; so that the more com- 

 pletely our attention is concentrated upon 

 the thing at which we specially wish to look, 

 the more distinctly we see it. The micro- 



