NOVEJIBER 1, 180i.] 



KNOWLEDGE. 



251 



from au undivided greyish spot to an archipelago of five 

 distinct and clearly separated markings. 



Again, in Flammarion's " Mars," a series of representa- 

 tions are given of Herschel II. Strait. First in order of 

 date come Beer and M;idler's drawings of 1830, in which 

 we see the " strait'' (Fig. 1), not as a strait, but as a 

 snake-like inlet, ending in a dark round spot. In 1862 

 Lockyer (Flammarion's "Mars,' p. 1.3.3i represents the 

 terminal spot as a nearly rectangular marking. In the 

 same year Kaiser gives the northern edge of this rectangular 

 spot a shaded appearance, as if he suspected the presence 

 of the two "estuaries" ilbid., p. 174). In 1864 (first 

 sketch of Fig. 2) Dawes resolves this rectangular spot into 

 the now well-known form of " Dawes' Forked Bay." In 

 1879 Schiaparelli (Flammarion's "Mars," p. 336) traces 

 two canals flowing into the two arms of the Forked Bay. 



These diS'erent aspects correspond precisely to the eflect 

 of improved " seeing," using that term in the larger sense 



Tig. 3. — Drawii'gj of ilivr?. hv Given, in 1^77. From I'lammariou'^ 

 •• Mars," p. 274. 



'adopted above. For if a careful drawing be made of the 

 district under the aspect which it presented to Schiaparelli 

 in 1879, and the drawing be approached from a distance, 

 it will assume in succession precisely the apparent phases 

 delineated by the above-named astronomers ; allowance 

 being made for the effect of foreshortening due to the 

 difference of presentation of the planet, and to the inevitable 

 inaccuracies of the drawings of even the most skilful 

 artists. 



I I ventured to lay stress, in my paper on " The Tenuity 

 of the Sim's Surroundings " in the March number of 

 Knowledge, on the fact, which we so easily overlook, that 

 i"the smallest portion of the sun's surface visible by us 

 as a separate entity, even as a mathematical point, is yet 

 really a widely extended area." The same truth applies 

 in its degree to the planet Mars. We have no right to 

 assume, and yet we do habitually assume, that our tele- 

 scopes reveal to us the ultimate structure of the sm-face of 

 the planet. 



1 An illustration of this point was afl'orded me some time 

 ago, when a question arose as to the hmit of visibility to 

 the naked eye of sunspots. I was astonished to find that 



a group of spots had been recorded as seen directly, when 

 their total area was much less than that of many well- 

 defined circular spots that had entirely escaped scrutiny. 

 A few experiments convinced me, however, that the 

 observation was perfectly correct, and that it was often 

 possible to see a straggling group of small unimportant 

 spots, when a single spot of considerably greater total area 

 would be invisible. 



I then tried how small an object could be detected 

 without optical assistance, the objects being always black 

 marks (Indian ink) on white glazed paper, illuminated by 

 dull diffused daylight. 



The limit of my vision for a circular dot ranged from a 

 diameter of 80 " to 36" of are. One of 20" was quite 

 invisible ; of 40', distinctly seen. This was decidedly 

 smaller than I had anticipated. But the limit for a straight 

 line, to my surprise, was as low as 7'' or 8 ' ; 12" w-as easy 

 and conspicuous. More than this, a pair of lines, each 

 only 4" in breadth, and the pair separated by say 20", was 

 visible as a faint single line ; two lines, even of only 3", 

 meeting at a very acute angle, were visible after their 

 separation had diminished below about 2.5". In each case 

 the object was unmistakably discenwd, and appeared as a 

 line or dot; it was not, of course, defined so as to be seen 

 in its true form. 



Further, a chain of dots, each of 20", irregularly disposed 

 along a straight line, the average interval between any two 

 dots being three times the diameter of a dot, was easily 

 seen as a continuous straight line, whilst a double chain 

 of yet smaller dots, each 4'' in diameter, and the two chains 

 some 40'' apart, was visible as a very faint continuous 

 line. 



The theoretical limit of visibility has been given as 40" 

 or a little greater, a limit with which tbe abive observations 

 are really in tolerable accord ; for when the angular 

 diameter of the object fell much below 40" it was seen, not 

 as a minute defined black dot, but as a grey diffused spot 

 of about 40" or 45" in diameter. It would seem, then, 

 that the smallest perceptible area is about 40", but tbat if 

 there be within any such given area a sufficiency of dark 

 markings, however individually miuute, to turn tbe white 

 to a decided grey, then that area will be visible as a grey 

 spot. Two lines or a number of dots, easily visible as one 

 object when close together, can readily be made invisible 

 by a greater angular separation. 



It seems to me that these rough experiments have a 

 decided bearing upon the "canal system" and the supposed 

 changes of Mars. It seems a violent hypothesis to call in 

 inundations extending over many thousands of square 

 miles to account for merely temporary changes, for sooner 

 or later the old districts take on the old configuration, more 

 especially since, as I have already shown, the meteorology 

 of Mars must necessarily be a languid one. Indeed, it may 

 happen that whilst several independent observers have 

 recorded a change, others equally skilled ha\e seen the 

 planet as before. 



But if what we see is not the ultimate structure of the 

 planet's surface, if, especially in the half-tone regions, like 

 the De la Rue Ocean, we have an intermingling of miuute 

 areas of dark and light — be they water and land, forest and 

 bare rock, prairie and sandy desert, or what you will — it is 

 easy to see how enormous changes may apparently occur 

 in a very little time. What we actually see is a greyish 

 spot, contrasted with dark spots but little darker than it 

 is, and with bright spots but little brighter. What is 

 required, then, in the observer is not so much keenness of 

 vision to detect minuteness of detail as power to appreciate 

 delicate difi'erences of tone. And the formation or dissipa- 

 tion of thin cirrus cloud above such a half-tone district wiU 



