Jttke 30, 1999.] 



SCIENCE. 



901 



eras not notably different from that of to- 

 day. The facts even justify the seemingly 

 extravagant statement that at several stages 

 in geological liistorj', earlj' and late, the 

 surface of the ancient ocean did not vary a 

 foot from that of the present, since it must 

 have passed both above and below the pres- 

 ent horizon repeatedly during the earth's 

 history. Geological evidence, therefore, 

 interpreted on its own legitimate basis, 

 seems to lend no appreciable support to any 

 theory that postulates a high speed of rota- 

 tion for the B&vlj earth, or a low speed of ro- 

 tation for the present earth, unless that 

 hj'pothesis is correlated with the assumption 

 of an almost perfect adjustability of the 

 form of the earth to the changing rotation, 

 in which case the argument of Lord Kelvin 

 set forth on p. 670 stands confessedly for 

 naught. 



If we postulate a slow accretion of the 

 earth and of the moon alike, the whole 

 subject of the former speed of rotation of 

 the earth and the relations of the earth to 

 the moon take on a new aspect and invite 

 investigation along the lines of new work- 

 ing hypotheses. Can it be shown that it is 

 absolutely necessary that the aggregating 

 meteoroids gave to the earth an exceedingly 

 high rotation at the outset ? Is not this as- 

 sumption of high rotation merely an off- 

 spring of the nebular hypothesis? If the 

 moon were aggregated slowly and came into 

 tidal functions at a late stage, and at a dis- 

 tance from the earth's center quite un- 

 known, may not all its relations to the 

 earth have developed on much more con- 

 servative lines than those worked out by 

 Darwin and at the same time preserve 

 those apparently significant relations to 

 the movements of the two bodies to which 

 Darwin has so strongly appealed in support 

 of his hypothesis of the histoiy of the two 

 bodies ? In other words, without challeng- 

 ing the validity of Darwin's most beautiful 

 investigation in the essentials of its method, 



may not a change in the premises dedu- 

 cible from an equally legitimate hypothesis 

 of the original condition of the two bodies 

 lead to results in equally satisfactory accord 

 with the existing relations of the two bodies'? 



At any rate, as remarked at the outset, 

 the time-limits assigned on tidal grounds 

 are not very restrictive, even on the as- 

 sumptions made, and when they shall be 

 worked out on revised data in accord with 

 the newer hypotheses they may, perhaps, 

 even be found to favor the longevity of the 

 earth and become one of the arguments in 

 support of it. 



T. C. Chambeklin. 



Univeesity of Chicago. 



(To he concluded.^ 



PEESPECTIVE ILLUSIONS FBO IT THE USE OF 

 MYOPIC GLASSES. 



The phenomena to be described occurred 

 during the iirst days' use of myopic glasses, 

 and may be grouped under the following 

 heads : 



a. There was an apparent diminution 

 in size of moving objects — persons, animals, 

 street cars — as compared with buildings, 

 natural scenery, and, in general, with the 

 elements of the background of the visual 

 field. Here the total visual fields of the 

 normal and of the myopic eye are equally 

 extensive ; there are the saoae number of 

 projection points ia each. Over thi.s back- 

 ground, in the case of a myopic individual, 

 there is distributed a relatively small num- 

 ber of distinct and at the same time inter- 

 esting or important olyects. When the 

 near-sighted person puts on powerful glasses 

 the number of such important and interest- 

 ing distinct objects thrown upon this back- 

 ground is vastly increased ; it is crowded 

 with a multitude of persons, animals, trees, 

 buildings, and the like. There are here two 

 sets of factors whose interpretation in terms 

 of perspective poiut in divergent directions. 

 Multiplicity of objects in the visual field 



