2go 



SCIENCE. 



over, as 180 in air is equal to i.o (since n = i.o, and 

 the sine of half i8o Q or 90 = 1.0), we see with equal 

 readiness whether the aperture of the objective'is smaller 

 or larger than that corresponding to 180 in air. 



Thus, suppose we desire to compare the relative apert- 

 ures of three objectives, one a dry objective, the second 

 a water-immersion, and the third an oil-immersion. 

 These would be compared on the angular aperture view 

 as, say, 74 air-angle, and 118 balsam-angle; so that a 

 calculation must be worked out to arrive at a due appre- 

 ciation of the actual relation between them. Applying, 

 however, "numerical " aperture, which gives .Co for the 

 dry objective, .90 for the water-immersion, and 1.30 for 

 the oil-immersion, their relative apertures are immedi- 

 ately appreciated, and it is seen, for instance, that the 

 aperture of the water-immersion is somewhat less than 

 that of a dry objective of 180 , and that the aperture of 

 the oil-immersion exceeds that of the latter by 30 . 



When these considerations have been appreciated, the 

 advantage possessed by immersion in comparison with 

 dry objectives is no longer obscured. Instead of this ad- 

 vantage consisting merely in increased working distance 

 or absence of correction-collar, it is seen that a wide- 

 angled immersion objective has a larger aperture than a 

 dry objective of the maximum angle of 180 ; so that for 

 any of the purposes for which aperture is desired, an im- 

 mersion must necessarily be preferred to a dry objective. 



The task of making an abstract of these papers was 

 not a light one and we are indebted to the Enghsh 

 Mechanics for the above resume. 



BOOKS RECEIVED. 



Discovery of the Preglacial Outlet of the 

 Basin of Lake Erie into that of Lake Onta- 

 rio ; with notes on the Origin of our Lower Great 

 Lakes. By Prof. J. W. Spencer, B. A. Sc., Ph. D., 

 F. G. S., Kings College, Windsor, N. S. 1881. 



As one new branch of knowledge is raised to a science, 

 there still seems to be some other rising to importance. 

 For a longtime the explanation of the Physical Features 

 of America has been handed over to the rival Glacier and 

 Iceberg theories, and though much good work has re- 

 sulted, yet an almost unlimited amount of nonsense has 

 been written, especially by the extreme or ultra-glacial 

 school. During all these years comparatively little atten- 

 tion has been given to the subject of the river geology, 

 more than that many buried channels have been recorded 

 with but few attempts at the reduction of the abstract facts 

 to a branch of Science. There has, however, been a very 

 great difficulty, owing to the Preglacial valleys often 

 being entirely obscured, or, if apparent, an absence of the 

 knowledge of their depths has prevented generalization. 

 In most of the cases recorded, the buried channels have 

 not had courses greatly differing from those of modern 

 times. It has been known for some time that the 

 waters of most of the great lakes had southern outlets 

 when at higher levels, and even to-day the drainage of 

 Chicago passes to the Mississippi. It has been frequently 

 suggested that Lake Ontario emptied by the Mohawk into 

 the Hudson. This, however, was not the case. We are 

 then compelled to place General G. K. Warren as the 

 father of Fluviatile Geology, for he discovered that the 

 Red River of the North (with Lake Winnipeg, the Sas- 

 katchewan, and other great rivers of the North West terri- 

 tories of Canada, as tributaries) discharged by the 

 Minnesota river into the Mississippi, and thus produced 

 a river to which no modern water is comparable. On 

 further investigation Gen. Warren's views are found to 

 require some modification, yet this does not detiact from 

 the position which may be fairly ass'gned to him. Dr. 

 Newbury's observations in Ohio have also thrown much 

 additional light on the subjed, but a much more im- 

 portant work has been accomplished by Mr. J. F. Carll, of 1 



Pennsylvania, when from a careful study of the levels and 

 borings for oil in that State, he discovered that the Upper 

 Alleghany and several other rivers now flowing into the 

 Ohio, formerly emptied into Lake Erie (or its basin). 



But the most important contribution on the subject 

 of Fluviatile Geology that has been made is the recent 

 paper of the above title, by Piofessor Spencer, now 

 of Kings College, Nova Scotia, but formerly residing 

 in the lake region, in the Province of Ontario. The 

 paper of the above title was read before the American 

 Philosophical Society, of Philadelphia, and its publi- 

 cation will be found in the forthcoming proceed- 

 ings of that Society. It is also being reprinted as an 

 appendix to Report Q 4 of the Pennsylvania Survey, as 

 shown by the maps which accompany the author's 

 edition, of which we have just received a copy. The fol- 

 lowing is a synopsis of the principal points of the paper: 

 The Niagara escarpment bends abruptly at the west- 

 ern end of Lake Ontario, and has a height of about 500 

 feet above the lake. Through this limestone ridge the 

 Dundas valley extends, and enters the extreme western 

 end of the lake. At the narrowest portion of the valley the 

 width is upwards of two miles, and the margins are those 

 of the walls of a perfect canon, 500 feet deep. But by 

 boring near one of its margins, the buried channel is 

 found to reach 227 feet below the surface of Lake Ontario, 

 making a total depth of 743 feet, but with a computed 

 depth in the central part of its course of not less than 

 1000 feet. The author first discovered that the ancient 

 upper portion of the Grand River left its modern course 

 south of Gait, and although a portion of the old bed is 

 entirely obscured, yet by pursuing the course of the deep 

 wells the ancient route can be traced through the drift to 

 the western end of the Dundas canon and Lake Ontario. 

 In following up this subject Dr. Spencer discovered that 

 the lower portion of the Grand River was formerly an 

 outlet of the Erie basin, which discharged by a course 

 Irom a point southward of Cayuga (Province of Ontario), 

 and flowed to the westward of this town and entered the 

 present valley, wnich is two miles wide and eighty feet 

 deep, but underlaid deeply with drift. Westward of 

 Seneca the ancient river left its modern course and passed 

 into the Dundas valley. All these observations are elabor 

 ately worked out by levels, deep well borings, and 

 deep ravines, with the one well in this course indicating 

 a depth of 1000 feet of drift in the ancient valley, measur- 

 ing from the limestone floor of the county. 



The outlet of Lake Erie is directly opposite to that of 

 the ancient Alleghany River. 



Again, Dr. Spencer has made a study of the sound- 

 ings of the lakes, and has discovered a long submerged 

 escarpment extending along the southern side of Lake 

 Ontario to near Oswego, at the foot of which the Ancient 

 River from the Dundas Valley ran. The author has 

 shown that an ancient, broad channel, extended from 

 Lake Huron and entered Lake Erie between Port Stan- 

 ley and Vienna, in the Canadian Province of Ontario. 

 This channel has a marginal depth of 200 feet below 

 Lake Erie, but with a probable depth sufficient to drain 

 Lake Huron. 



With regard to Lake Superior, Prof. Spencer shows 

 that it formerly emptied into the northern end of Lake 

 Michigan, and formed a river channel now represented 

 by deep pot-holes. He brings forward some ot the evi- 

 dence showing that Lake Michigan emptied or was com- 

 pletely drained by the tributaries of the Mississippi, and 

 that this lake was probably disconnected from Lake Hu- 

 ron. At the same time, he shows that Lake Superior 

 (when it was at no higher level than at present) did not 

 empty by the Green Bay and valley of the Fox and Wis- 

 consin Rivers. 



The author denies the hypothesis of the glacial origin 

 of the Great Lakes, and brings forward strong evidence 

 in support of his views. He correlates with his work and 

 1 maps the buried channels discovered in Pennsylvania and 



