December 23, 1892. J 



SCIENCE. 



355 



objective is down to the axis about which the mirror rotates. 

 Use the plane side of your mirror, and reflect the image of a dis- 

 tant object, eg., a dot on a window-pane, which must also be in 

 the line of the axis of the mirror's rotation, into the field of view 

 so that the aforesaid dot will appear in the centre if the mirror is 

 directly beneath. 



If all is properly adjusted, on moving the mirror-bar to and fro, 

 the dot will always appear reflected from the centre of the mirror. 

 The angle through which the mirror-bar must be swung, so that 

 the image of the dot may be first where the apex of one branch 

 of the hyperbola was and then where the other apex was, will be 

 the optical angle in air (2E). The angle, vs^hich may be swung 

 from where the dot disappears on one side to where it disappears 

 on the other, will give the angular aperture of the objective used. 



SUBDIVISIONS OF THE AZOIC AEOHiEAN IN NORTHERN 

 MICHIGAN. 



BY M. E. WADSWORTH, STATE GEOLOGIST, HOUGHTON, MICH. 



The work of the Michigan Geological Survey in 1890 made it 

 clear that the Azoic system of the Lake Superior district of 

 northern Michigan was composed of at least three unconformable 

 formations. This conclusion was published by me early in 1891, 

 in an article. entitled "A Sketch of the Geology of the Marquette 

 and Keweenawan Districts," which was appended to a pamphlet 

 called "Lake Superior — Along the South Shore," New York, 1891. 

 These general conclusions have been confirmed by the work of 

 the two subsequent seasons, and two other unconformable forma- 

 tions rendered probable, although not yet proved conclusively. 

 A discussion of these points will subsequently be given in detail 

 in the reports of the State geologist. 



The following are the formations as made out and named from 

 prominent localities by the Michigan Survey, commencing with 

 the oldest. There are given with this the formations as deter- 

 mined by the United States Geological Survey, showing their 

 aupposied equivalency. 



POTATO SCAB. 



BY H. L. BOLLBY, FARGO, NO. DAKOTA. 



This disease of the potato-tuber need not longer mar the ap- 

 pearance, quality, and economic value of the crop. The corro- 

 sive sublimate treatment, recommended in Bulletin No. 4, of 

 North Dakota Experiment Station, has had another year's trial, 

 not only at this station but by potato-growers, and it has again 

 proven effective in a degree beyond previous expectation. 

 Though the damage done each year by this disease is so great 

 and widespread in its occurrence, no plant-disease now success- 

 fully combated is so easily prevented and at such a slight cost. 



As indicated in my work of previous seasons, it is again demon- 

 strated this past season that the damage to the yield of the crop 

 is very much in excess of that usually supposed. The disease at- 

 tacks not only the tubers but the base of the vines also; the result 

 is a shorter-lived vine, a greater number of potatoes set upon the 

 vines than there would be under normal conditions, and, third, 

 much smaller tubers. The second of these statements, perhaps, 

 needs some explanation. I have only one to offer, which I think 

 to be about the correct one, i.e., the disease early attacks the 

 young tubers as they set upon the vines, normal growth is 



checked, and the excess of vigor in the vinos which is at its 

 highest stage at this time finds expression in the formation of 

 new tubers to the further detriment of those already set upon the 

 stems. 



The results of this summer's work when averaged for all tests 

 show an average gain of one-half pound per hill in favor of the 

 treated seed-tubers, and 99.33 per cent of total product void of 

 disease; while untreated seed in all cases gave a product in 

 which less than one per cent undiseased tubers were found. 



The cost of treatment for a crop of two or three acres of pota- 

 toes need not exceed fifty cents, aside from that of one or two 

 days' extra labor. 



Details of the season's work will soon appear in bulletin form. 



The fourteenth edition of Franklin Leonard Pope's "Modern 

 Practice of the Electric Telegraph " has just appeared (D. Van 

 Nostrand Company, 334 p., 8°, |1.50). This is a technical hand- 

 book for electricians, managers, and operators, and contains 185 

 illustrations. As this book has been a standard authority on 

 telegraphy for almost a quarter of a century, it is unnecessary to 

 say anything to those already familiar with it, except that it has 

 been rewritten, enlarged, and brought to date. To others, it is 

 sufficient to repeat that this is the fourteenth edition of a compre- 

 hensive and handsomely illustrated work. 



— The Johns Hopkins University has published a large geologi- 

 cal map of the vicinity of Baltimore, prepared by Professor G. H. 

 Williams. It is an extension of a similar map prepared by Wil- 

 liams and Darton and published by the U. S. Geological Survey 

 some months ago; but the coloring on the present issue departs 

 from the color system adopted for the Survey maps; as if experi- 

 mentally to try another device, better applicable to the region 

 here included. The experiment is successful; but it does not 

 follow from this isolated instance that the color scheme of the 

 Survey should be changed in its uniform application to the great 

 series of maps for which it was prepared. 



— The Geological Survey of Missouri, Arthur Winslow, director, 

 has issued an atlas report on the Higginsville map-sheet, with 

 two colored maps and several papes of text, chiefly concerning 

 the coal measures of the region. The large form of the report 

 avoids the difficulty of folding the maps, which constitute its 

 chief feature; but the size of the atlas is inconveniently large for 

 easy keeping in the library or on the desk. As a forerunner, 

 how.ever, of what we shall have to deal with when the publica- 

 tion of similar atlases of maps and texts is begun by the U. S. 

 Geological Survey, the Higginsville alias gives us useful practice. 

 The extension of such a series of reports to include the other map- 

 sheets of Missouri will add greatly to our information about that 

 State. 



— Professor F. A. Forel of Morges, Switzerland, has long been 

 known as a careful investigator of the natural history of Lake 

 Geneva, on whose borders he was born and bred. Several years 

 ago he published a small pocket volume on the various features 

 of the lake; and now we receive from Rouge of Lausanne the first 

 of three volumes, entitled "Le Leman; Monographic Limnolo- 

 gique." Judging the whole work by what now appears, as well 

 as by the high standard of Forel's pi-evious writings, it is not too 

 much to say that it will take an eminent place among scientific 

 monographs, ranking with Favre's " Alps of Savoy" and Heim's 

 "Mechanism of Mountain-Making." Thedivisionof the present vol- 

 ume which willexcite most general interest is the serious discussion 

 of the origin of the lake, from which Forel concludes that, of all 

 suggestions, the one which ascribes the basin to the warping of a 

 pre-existent valley is by far the most probable. The winds of the 

 lake are fully described ; but in the excursions in connection with 

 them into the theory of c.vclones and anticyclones, the author 

 evidently ventures into a field somewhat unfamiliar to him; his 

 suggestions on this subject not being made in view of all that is 

 known about it. The second volume will include an account of 

 the seiches, or oscillations, of which Forel has already written so 

 much; the third will discuss the biology of the lake and its human 

 antiquities. An excellent map accompanies the first volume. 



