O.t. 1st, 18S7.] 



SCIENTIFIC NEWS. 



185 



very little to do with refraction, which depends on the tem- 

 perature and pressure of the atmosphere, but on no optical 

 principles can it be shown how a mist or fog could produce 

 a magnifying of an object. We must therefore fall back 

 on the effect of the surroundings, and here Descartes' 

 theory probably solves this problem better than the one to 

 which reference has been made. When travellers who 

 have lost their way, peering anxiously into the blankness 

 of a mist, catch the first glimpse of an object, they are in- 

 clined to suppose that because it has only just come into view, 

 it is a great way otT, and are sometimes startled by its colossal 

 appearance. 



As another example of the difficulty of forming an accu- 

 rate estimate of the apparent size of an object, it is found 

 that most people imagine that an opera-glass which only 

 magnifies some two or three times does not magnify at all, 

 b'.it merely makes the image clearer. The magnifying 

 power can easily be appreciated by holding the glass to one 

 eye only, keeping both eyes open ; the apparent size of 

 the object seen by direct vision and by the glass can then be 

 compared. 



It is to be observed that these errors all relate to the es- 

 timation of absolute size, based upon the apparent or an- 

 gular magnitude of the thing observed, and a conscious or 

 unconscious estimate of its distance from the observer. 

 The judgment of relative size can be made much more ac- 

 curately. Among the interesting measurements made at 

 the various anthropometric laboratories which have been 

 arranged by Mr. Francis Gallon is that of the judgment of 

 the eye in dividing a line of fifteen inches into three parts 

 and into two parts, and in setting a hinged rod square with 

 a fixed rod. Such estimates are of course made with 

 greater facility and accuracj' by those engaged in occupa- 

 tions of a more or less structural or mechanical nature, but 

 the errors of decidedly unskilful persons in the judgment of 

 such relative magnitudes are of an entirely difterent order, 

 and far less serious than those of many scientific observers 

 in questions of mere apparent size. 



It may be well, in conclusion, to give the actual dimen- 

 sions of the sun and moon, and their distances. The 

 moon is about 2,159 niiles in diameter, and its distance from 

 the earth's centre varies from 252,948 to 221,593 miles. 

 Its diameter is to its mean distance nearly as 9 is to 1,000. 

 The diameter of the sun is 852,000 miles, and its distance 

 varies from 92,963,000 miles in summer, to 89,897,000 in 

 winter. The proportions between the diameters and the 

 distances are so close in the two cases that the apparent 

 diameters are nearly the same. The sun only varies by 

 about one minute, while the moon varies about four min- 

 utes. The mean apparent diameter of the sun is nearly 

 one minute greater than that of the moon. If a total 

 eclipse occurs when the moon happens to be of greater ap- 

 parent diameter than the sun, the latter is completely hid- 

 den, but if otherwise, the eclipse is said to be annular, 

 because an annulus or ring of light is to be seen round the 

 moon. 



In the recent total eclipse the disc of the moon was con- 

 siderably greater than that of the sun, and it was hoped 

 that observations taken at northern stations, where the sun 

 was only just hidden at one edge, compared with those at 

 southern stations where the opposite edge or limb was just 

 eclipsed, would have thrown some further light on the 

 measurement of their apparent diameters ; the necessary 

 observation being the duration of the totality. Unfor- 

 tunately cloudy weather prevailed so generally, that it is 

 probable that no such calculations will be possible. 



CORROSION OF METALS. 



MUCH trouble is often experienced in mines and other 

 underground works owing to the rapid corrosion of 

 the iron and steel used in the machinery. This corrosion is 

 chiefly due to the acid character of the water in the mines, 

 and in some places it is very difficult to contend with. 

 Brass and gun-metal resist the corrosive action to a great ex- 

 tent, but they are not sufficiently durable. Of late an alloy 

 called " Delta metal," which is a brass as hard and durable 

 as mild steel, has been much used for various kinds of ma- 

 chinery and fittings, and recently an interesting trial has 

 been made to test its power of resisting corrosion in the 



acid waters of a mine. Equal-sized bars ot wrought iron, 

 steel, and Delta metal were carefully weighed and then left six 

 and a half months in the mine water, and at the end of that 

 time they v.-ere taken out and reweighed. The wrought 

 iron had lost 46-3 per cent., the steel 45-4 per cent, and the 

 Delta metal only 1-2 per cent, of its original weight. The 

 bars were photographed on being taken out of the water, 

 and from the accompanying illustrations it will be seen at a 

 glance that there is a very marked difference in the effects 

 produced on the three metals. 



How TO Copy Printed Matter. — Printed matter may be 

 copied on any paper of an absorbent nature by dampening 

 the surface with a weak solution of acetate of iron and 

 pressing in an ordinary copying press. Old writing may also 

 he copied on unsized paper if wet with a weak solution of 

 sulphate of iron mixed with a simple solution of sugar 

 syrup. 



