July 4, 1884.] 



♦ KNOW^LEDGE ♦ 



9 



This last process has been termed multiplication by en- 

 dogonidia ; and we cannot but fancy that it is to its potent 

 agency that we are indebted for the myriads of germ 

 forms which pollute the atmosphere. The inhalation of 

 these germs, however, does not seem to be all-powerful in 

 the spread of infections, as they are to a large measure in- 

 tercepted by the natural filter, commonly known as the 

 nostrils, and are further checked in their growth by the 

 unfavourable secretions and gases given off by the glandu- 

 lar organs of our bodies ; so that in the struggle for ex- 

 istence they are compelled to succumb, and their harmful 

 action is thus arrested, or reduced to a minimum. The 

 "nervous" or other pathological condition of the individual, 

 of course, opens the gates even to these dry germs ; but it is 

 averred that persons in a normal state of health are able, 

 at most times, to withstand their ravages. 



When, however, they gain access to their hotbed of 

 water, their activity is so stimulated that the poor unfor- 

 tunate who gives them a reception must inevitably become 

 a prey to these poisonous parasites. It thus becomes in- 

 cumbent on all those to whom the welfare of their fellow- 

 beings is entrusted, to the heads of families, as much as to 

 water-supply companies, to learn something about these 

 matters, and to seek for remedies, such as may lie in their 

 power, which shall effectually prevent the spread of disease 

 and the increase of mortality. 



In this paper it has been our endeavour to show how 

 seemingly insignificant yet subtle an influence, — an in- 

 fluence which can baffle the chemical or even the practised 

 microscopical detective, — is at work undermining the con- 

 stitutions of thousands of healthy lives. We shall in 

 continuation of this subject show how those evils might be 

 remedied, and what has already been done by practical 

 workers in this field of inquiry. Our next communication 

 will deal with the phy.siographical aspect of the water- 

 supply question, suitably illustrated. 



ON PECULIARITIES OF SIGHT, AND 

 OPTICAL ILLUSIONS.* 



By Natii. E Greek. 



A MOST interesting and instructive paper might be 

 written on the subject of optical illusions in connec- 

 tion with astronomical observation. How many minute 

 companions to stars, how many extra satellites of planets, 

 how many strange appearances, have been seen by certain 

 observers, and by them only. Yet those observers believed 

 in their own sight, and were fully convinced of the 

 actuality of the appearances reported. May not an ex- 

 planation be found in the eccentricities of optics or in 

 personalities of vision 1 



The object of this paper is to draw attention to the 

 subject, and by a few confessions to induce others to take 

 it up, being assured that, although the individual may feel 

 depressed, the great cause of truth will be advanced by the 

 discussion. 



And first, with regard to minute points of light. The 

 writer has experienced a difliculty when searching for faint 

 ■somites, or the inner satellites of Saturn, in decermining 

 which are real and which spurious points. The eye having 

 a tendency to produce these where they do not exist, aud 

 the real things being only seen by glimpses, it requires great 

 patience, and frequent repetition of a point in the same 

 position to be assured of its existence. A friend once re- 



* From the Astronomical Register. 



marked when searching for the satellites of Saturn, " How- 

 many do you want me to see, for I can see as many as you 

 like." Surely this optical illusion will account for the ap- 

 pearance of many minute object?, that others have been 

 unable to confirm. Another illusion is the persistence of 

 an object on the retina after the eye has been withdrawn 

 from the telescope. This has occurred after long observa- 

 tion of Jupiter with an 18-in. reflector. On removing the 

 eye from the instrument, to make a drawing of what has 

 been observed, a clear, bright disc has so interfered between 

 the eye and the pencil-point, that an effort was required to 

 remove the illusion before the drawing could be made. May 

 not this retention of an object on which the eye has long 

 been fixed account for the appearance of a satellite through 

 the limb of Jupiter, the image of the satellite being con- 

 tinued on the retina after it had passed behind the 

 planet ? * 



But what shall we say of the reported visibility of the 

 unillumined portion of Venus % This phenomenon has been 

 reported so frequently that it seems like heresy to doubt 

 the fact. We are not referring to the fine ring of light 

 that surrounds the disc just before immersion at a transit, 

 or when Venus is very near the sun at inferior conjunction, 

 but to those reported appearances, in full daylight, when 

 the planet is at greatest elongation. The writer had a 

 friend who could always see the dark side, even in the 

 finder ; he has shown Venus to another unaccustomed to 

 astronomical observation, when the planet was on the 

 meridian and about half full. " How is it," said he, " that 

 I see the whole of the round ? " And very recently, when 

 friends were in the observatory, and viewing Venus, on 

 putting the question, " Can you see the unillumined por- 

 tion ?" the answer has been, " Now you mention it, I think 

 I can." In all these cases, and many others, the writer has 

 been unable to perceive that which others have seen. 



One more illusion, and we have done. When observing 

 the ring of Saturn, and especially the outer ring, for evi- 

 dences of division, a very distinct shadow of Saturn has 

 appeared on the following portion of the sky — this is, of 

 course, a simple optical defect, arising possibly from the 

 same cause as the dark forms that are seen after looking 

 at the sun. It is just possible that a similar effect follows 

 Venus, and to some eyes appears like the dark portion of 

 the planet against the sky. f 



There is a fine opportunity before us of testing some of 

 these appearances, and the readers of this journal may be 

 induced to record their experiences. 



DrRiXG the year 1883, Professor Simon Xewcomb, U.S.X., 

 visited Europe, by order of the United States Government, for the 

 purpose of collecting information respecting the most recent im- 

 provements in astronomical instruments. His report to the Secre- 

 tary of the Xavy contains a good deal of valuable information for 

 the practical astronomer. He speaks highly of the definition of 

 the enormous refractor at Vienna. He also commends the method 

 of supporting mirrors devised by the Mil. Henry of Paris, adding, 

 however, that it has, so far, not been employed with reflectors ex- 

 ceeding 12 in. in aperture. He by no means speaks with unmixed 

 admiration of the much beptiffed "Equatorial circle"; while he 

 has little but praise for the Strasburg circle under construction by 

 the Messrs. Eepaold. One of the most interesting things described 

 by Professor Newcomb is the extremely shallow form of mer- 

 cury basin employed for reflection - observations at Strasbnrg, 

 Leyden, <&c. 



* This is, however, inconsistent with the accounts given by Mr. 

 Todd, of Adelaide, and his assistants. — B. P. 



t This seems to me just impossible. The dark shade seen in this 

 way is of the same shape as the luminous object, and similarly 

 situated : the *' terminator " of Venus is not of the same shape as 

 the outline of the unillumined part of the disc. — B. P. 



