88 



KNOWLEDGE & SCIENTIFIC NEWS. 



[May, 1904. 



the known and natural methods of explaining a pheno- 

 menon. My inference is one based on the observed 

 effects of known causes; Mr. Lowell's inference is an ex- 

 cursion into fairyland. 



We know that the smallest single dark marking on a 

 bright ground which can be seen by an observer of per- 

 fect sight, without optical assistance, must have a 

 diameter of at least 34 seconds of arc. This diameter 

 depends upon the size of the rods and cones of the eye 

 which receive the visual impression, and compose the 

 sensitive screen. It is therefore an inevitable limit. .\s 

 this diameter is necessary for the object to be merely 

 perceived, or, in other words, to create any sensation at 

 at all, it follows that in order that the actual shape of the 

 object may be recognised, its diameter must consider- 

 ably exceed this limit, otherwise it will be seen as a truly 

 circular dot, whatever its actual shape. 



This is the case for small isolated markings just 

 within tlie limit of visibility. The case is different for 

 extremely elongated markings ; the increased length of 

 a marking will compensate for diminished breadth up to 

 a certain limit, but not beyond it. For a Ime of indefi- 

 nite length the limit of breadth approaches two seconds of 

 arc. A line of a breadth below one second of arc is 

 invisible, no matter what its length ; but it must have a 

 breadth many times this amount before it can be seen as 

 anything else than a mere line — before irregularities in 

 shape and breadth can make themselves apparent. 



In naked-eye vision, therefore, there is a considerable 

 range within which small objects, whatever their true 

 shape or nature, can only be seen as dots or as lines. 

 The result is that these two forms are certain to come 

 in evidence whenever we are dealing with objects too 

 minute to be fully and properly defined. 



The problem becomes more complicated when we are 

 using optical assistance, as there is a limit of definition 

 belonging to the telescope as well as to the eye. But 

 the principle remains the same ; the result of adding the 

 limitation of the telescope to the limitation of the eye 

 being that the actual magnification of the telescope can 

 never be nearly as effective as it is nominally. A power 

 of 300 on the best telescope in existence, and under the 

 best atmospheric conditions, would never show the 

 features ot the moon as distinctly as they would be seen 

 if the moon were brought 300 times as near. 



Mr. Story and Mr. Lowell both object that terrestrial 

 (or, as they are more usually called, " laboratory ") expe- 

 riments are altogether beside the mark when applied to 

 the interpretation of astronomical observations. The 

 contention is a ridiculous one, and if logically applied 

 would render it impossible to determine the instrumental 

 errors of a transit circle by the use of meridian marks, 

 collimators, or mercury trough, or the personal equation 

 of an observer, except by actual stellar observation. 

 They would also foibid us to identify the lines of solar 

 or stellar spectra by comparison with those of any terres- 

 trial element. 



Hut since it is contended that Mars alone can give us 

 valid information on the subject, to Mars let us refer. 

 If we turn to the drawings made by Beer and Miidler in 

 1830, two small objects exceedingly like one another 

 appear repeatedly. These are two dark circular spots, 

 the one isolated, the other at the end of a gently curved 

 line. Both recall the "oases" which figure so largely 

 in many of Mr. Lowell's drawings, and the curved line 

 at the termination of which one of the spots appears, is 

 not unlike the representation which has been given of 

 several of the " canals." There can be no doubt that in 

 the year 1830 no better drawings of Mars had appeared 

 than those to which 1 have referred, and that in 



representing these two spots as truly circular Beer and 

 Madler portrayed the planet as they best saw it. The 

 one marking we call to-day the Lacus Solis, the other the 

 Siinis Siihifus, and we can trace the gradual growth of our 

 knowledge of both markings from 1830 up to the present 

 time. The accompanying sketches of the same region 



Fig. 2. — Sinus 5aba;us and Lacus Solis. 



Sinus Sab.-eus 



Lacus Solis 



Beer and Miidler i»30. 

 Lockyer . . 1862. 

 SchiapareMi . . 1890. 

 Beer and Miidler 1S31. 

 Lockyer .. 1S62. 

 SchiaparcUi .. i8go. 



by Lockyer, in 1862, and by Schiaparelli, in 1890, illus- 

 trate well how the character of the markings revealed 

 themselves with increased telescopic power and experi- 

 ence in the observer. 



•' At first it seemed a little speck 

 And then it seemed a mist, 

 It mo\ ed and moved and tooli at last 

 A certain shape I wist. 



A speclv, a mist, a shape." 



If Beer and Madler, in 1830, had argued that the precise 

 circularity of these two spots, as they appeared to them, 

 was proof that they were artificial in origin, would they 

 have been correct? Would not the answer have been 

 valid that a spot too" small to be defined must appear 

 circular, and that, therefore, the apparent circularity pro- 

 bably covered detail of an altogether different form ? 

 We know that it would. Yet it is that same argument 

 in a far stronger form against which Mr. Lowell and 

 Mr. Story are contending to-day. Beer and Madler only 

 drew two of these spots ; Lowell shows over sixty. 

 Beer and Madler's two spots seemed to them precisely 

 alike; how utterly different those two spots appear to us 

 to-day the diagram may serve imperfectly to indicate. 

 Mr. Lowell's sixty or more " oases," with one or two 

 exceptions, appear all of the same character. Will any- 

 one dream that if the next seventy years brings telescopic 

 development equal to that shown in the last seventy, the 

 present uniformity of Lowell's "oases" will persist, any 

 more than the likeness of the two spots observed by Beer 

 and Madler? We need not even wait for the seventy 

 years. Up to the present moment I have carefully 

 avoided anything like criticism of the drawings of any 

 observer of Mars. I have repeatedly stated that I ac- 

 cepted them as being both faithful and skillul representa- 

 tions of what the observers saw. Ijut it is necessary 

 here to point out that the extreme simplicity of type of 



