February 17, 1893. J 



SCIENCE. 



93 



seen, is also appreciably darkened by its presence. When the 

 star is black and the card behind it white, the illusion is still 

 present, though a brightening of the black takes the place of the 

 darkening just mentioned. 



The rationaJe of the illusion is easy. The outer ring is really 

 transparent, and the edge of the card is really seen through it. 

 The transparency of the ring strongly suggests the transparency 

 of the centre, a suggestion that we accept the more easily because 

 the rapid rotation changes the appearance of the central portion 

 somewhat froQi its familiar resting appearance. The apparent 

 darkening of the portion of the central part overlying the black 

 card is to be similarly explained. It is especially interesting, 

 however, as being clearly a psychological illusion, an "illusion 

 of judgment," while the color illusions formerly so called are 

 being shown to be physiological, and largely, if not entirely, due 

 to the mutual influence of adjacent portions of the retina. 



A physiological explanation of this illusion seems hardly possi- 

 ble : and its psychical character is further attested by the ease 

 with which it is corrected when the card remains stationary, and 

 the unequal degree in which it seems to affect different observers. 



A SUGGESTION AS TO TOPOGRAPHIC MAPS. 



BY ARTHUR P. DAVIS, LOS ANGELES, CALIFORNIA. 



While the scientific and technical bodies of California and 

 other States are agitating the question of topographic maps, edu- 

 cating the public on the question, and endeavoring to secure 

 State appropriations and Government cooperation for such sur- 

 veys, it may not be amiss to suggest other sources that might be 

 made to furnish valuable contributions toward the same end. I 

 refer to the very extensive and cosily surveys made by engineers- 

 promoters, and companies to determine the necessary informa- 

 tion for the construction of railroads, iriigation s> stems, etc. 



I have known cases where a large number of preliminary or 

 trial lines have been run, the aggregate cost of which amounted 

 to many times that of a good topographic map of the region 

 under investigation, which would have shown all that the afore 

 said surveys can show, and a great deal beside ; for it is always 

 an open question, whether the line iinally determined upon is in 

 reality the best in all respects, or whether some other, of the 

 many untried possible routes might not be somewhat better or 

 cheaper. All thesepossibleroutes would be shown by a good con- 

 tour topographic map, and in addition thereto it would serve as a 

 valuable piece of data for any future additions or alterations in 

 the work. 



Another argument in favor of the contour map that ought to 

 weigh strongly in its favor with the persons above referred to, is 

 the philanthropic one, that such a map is a valuable and perma- 

 nent contribution to science. A large percentage of the scientific 

 discoveries that have contributed so prodigiously to the intellec- 

 tual and material advancement of the human race, have been 

 made by persons working without the incentive of financial gain, 

 and it is safe to say that few of them would have been made, if 

 that had been the only incentive followed. 



I do not believe that engineers and capitalists in charge would 

 prove less public spirited than others if their attention were 

 properly directed to this matter; and if geologists and others ac- 

 tively interested would persistently present these facts to the 

 proper persons, great good might result. The main point to be 

 carried is the substitution of the plane table far the transit in 

 making preliminary surveys. 



Without a systematic method of accurate field-sketching, which 

 is the essential principle of the plane table, topographic mapping 

 on any extended scale is impossible. Properly handled, with 

 triangulations to check locations, and level bench-marks to check 

 elevations, and with stadia to assist in sketching, plane table 

 work may be entirely accurate within any scale adopted, and 

 serve not only for preliminary information upon which to locate 

 routes for canals, railroads, etc., but is a permanent record of 

 comprehensive information to guide all future engineering opera- 

 tions in the country included, so that ordinarily at least two or 

 three times as much might be advantageously spent on the con- 



struction of contour maps as would be required for running pre- 

 liminary lines, and still the company would profit by the substi- 

 tution. If the labor expended upon the tremendous mass of 

 material now on record in the great engineering offices through- 

 out the west had been judiciously supplemented by field-sketching 

 on the plane table, a very great addition would have been made 

 to our topographic knovvledge, and I believe that such results 

 could be brought about by well-directed efforts on the part of the 

 proper persons. 



LETTERS TO THE EDITOR. 



*♦« Correspondents are requested to be as brief as possible. The writer's name 

 is in all cases required as proof of good faith. 



On request in a.dvance, one hundred copies of the number containing his 

 com,munication loill he furnished free to any correspondent. 



The editor will be glad to publish any queries consonant with the character 

 of the journal. 



Pseudoaurorae. 



Dr. Hatch's description of " Pseu loaurora Borealis " in your 

 issue of Dec. 2, 1893, calls to mind a peculiar phenomenon which 

 I once witnessed here, and which may have some bearing on 

 pseudoaurorae in general if not on the particular species observed 

 by Dr. Hatch. 



About three years ago, as I was returning from the business 

 part of the village of Plattsburgh, N. Y., my attention was taken 

 by two long, white, brilliant, quivering streamers in the south- 

 west, which at times seemed to shoot up and nearly reach the 

 zenith. This was an unusual direction for such a display, and I 

 at once turned toward the north to note its character there. 

 Buildings prevented a good view, bui I saw several stieamers 

 though none so brilliant as those in the south-west. Hastily 

 taking the phenomenon for a true display of the aurora. I hur- 

 ried home, only noting on my way that the streamers were 

 brighter now in one direction, and now in another, it was not 

 until I had called others out to witness the display, and remained 

 quiet myself for a moment, that I discovered that one very bril- 

 liant streamer seemed to be situated directly back of the known 

 position of one of our arc lamps, and what was still more curious 

 it refused to move from that suspicious position. This streamer 

 varied remarkably in brightness, now being short and faint, and 

 again long and brilliant. Along with these greater changes in 

 intensity there were more rapid and lesser changes, and in addi- 

 tion to these a constant shimmering of the light. There were 

 also slow wave-movements of brighter portions which ran from 

 below upward, or crossed the streamer from left to right. It 

 was movements of this nature, so like the curtain movements of 

 the true aurora, that led me for the moment to refer the phe- 

 nomenon to the aurora itself, and the many beams, which some- 

 times ran so high as to suggest a corona overhead, and which 

 varied in relative intensity if one was moving about, only helped 

 to confirm the error. Plattsburgh has had electric street-lamps 

 for seven years or more, yet this one display stands practically 

 alone by itself. The streamer which I studied most was over a 

 lamp something more than a thousand feet away, and was viewed 

 across some village lots with many buildings clustered around the 

 position of the lamp. The lamp itself was thus hidden from 

 view, though it hung over the centre of the street and could cast 

 no high shadows save alone those of the top of the lamp and its 

 wires. These, of course, helped to cut up the beam of light. I 

 do not think that a dark arch was present, though I can conceive 

 that one of Professor Hazen's shadows might have been thrown 

 by a group of trees or buildings in such a manner as to pioduce 

 one, had some convenient cloud been situated in the background. 

 I attribute the phenomenon to the peculiar condition of the at- 

 mosphere at the time. We were in a very light frost-fog and the 

 vertical and lateral movements of brighter waves were probably 

 due to denser portions of this frost-cloud, drifting along with the 

 air-currents. The varying light of the arc lamps served to make 

 these movements seem the more complex. I think it very proba- 

 ble that color was in many places present, due to a halo encircling 

 each lamp, although I do not recall having noticed it. From Dr. 

 Hatch's description it would seem that the two phenomena are 

 not the same, as when be retired from the lamps the " aurora " 



