620 



SCIENCE. 



[N. S. Vol. IV. No. 95. 



cally it would probably increase as the subdi- 

 vision of arcs is carried on. There are some 

 statements in the note referred to, which appear 

 to be misleading. To quote : 



" Even if the division error of any given line 

 could be determined with complete precision 

 with the telescope pointed at the zenith, this 

 division error would not hold true when the 

 telescope is pointed elsewhere. Nor is this 

 brought about by flexure alone. It is found 

 that if we determine the division errors of a 

 straight scale, these errors are completely 

 changed when the scale is reversed end for end. 

 No doubt unavoidable difference in the illumina- 

 tion and the eye of the observer are responsi- 

 ble for these unfortunate facts. But facts they 

 are, and the cause of much wasted labor. ' ' 



While the measurement of the division errors 

 of a straight scale might not hold true if the 

 scale were reversed end for end, this apparently 

 damaging condition has nothing whatever to 

 do with the measurement of the division errors 

 of such a divided circle, since it cannot possibly 

 be reversed ; but is always read, facing it, in the 

 same position. 



As the instrument is moved to various set- 

 tings, any single division passes under the vari- 

 ous microscopes, and is read at various inclina- 

 tions to the vertical, under various conditions 

 of illumination, and to make the illustration as 

 wide as possible, by various observers. The 

 reading at any microscope will be affected by 

 all the conditions of phase of that microscope, 

 and by the personal equation of the observer, 

 which may be, and probably is, peculiar to that 

 microscope. 



But there is no reason that these conditions 

 should differ for the various divisions, which 

 come in succession under the same microscope, 

 or set of microscopes. In every case of star 

 observation, or of determination of graduation 

 error, the difference is measured between a 

 reading of the circle at the required setting, 

 and some standard reading. 



Personal equation and phase should affect 

 each reading alike, and should be eliminated 

 from the results. 



As to the effects of differences in the illumi- 

 nation and the eye of the observer, if they 

 exist, they must be equally injurious to all ob- 



servation with this instrument, as, in fact, they 

 must be in every class of observing. Such 

 sources of error fall within the class admitted 

 as accidental ; with proper care and well de- 

 signed illumination, they are not believed to be 

 large enough to invalidate the results obtained 

 with fine instruments in astronomy of precis- 

 ion. R. H. Tucker, 

 Lick Obseevatoey. 



SCIENTIFIC LITERATURE. 

 Elements of Geology, a text-book for colleges 

 and for the general reader. By Joseph Le 

 CoNTE. Fourth edition, revised and en- 

 larged, with new plates and illustrations. 

 New York, D. Appleton & Co. 1896. 

 For nearly twenty years Le Conte's Elements 

 of Geology has stood side by side with Dana's 

 Manual in the working libraries of American 

 geologists and teachers. It has found equal 

 favor in the class-room and the study room, 

 and has been widely read by the cultured lay- 

 man. Holding this enviable position, it needs 

 neither introduction, encomium nor criticism ; 

 but the appearance of a new edition may rather 

 serve as an occasion to enquire what are the 

 qualities on which its success depends. 



I conceive that one of the first of these is a 

 wise choice of material. The author is fortu- 

 nate in possessing the power to select the more 

 essential, and ignore the less essential, so that 

 the principles he expounds are not obscured by 

 clouds of detail. Moreover, he devotes all his 

 space to his proper theme, the science of geol- 

 ogy, assuming, on one hand, that the reader 

 has all necessary knowledge of physics, chem- 

 istry, astronomy, meteorology, biology, and 

 even mineralogy, and not undertaking, on the 

 other, to teach him either the technology of the 

 professional geologist or the economic applica- 

 tion of geologic results. 



Of equal importance, perhaps, is the order of 

 presentation, which deviates somewhat from 

 the strictest system so that it may follow lines 

 of least resistance. One who writes on a com- 

 plex subject is always embarrassed by the fact 

 that the easy explanation of each part seems to 

 require the previous explanation of some other 

 part ; and in geology this contest for priority 

 lies between processes of change and the struc- 



