July, 1906] 



KNOWLEDGE & SCIENTIFIC NEWS. 



477 



uppermost, is accepted as strong evidence tliat the 

 forest did not grow on its present site. There is the 

 further argument that the number of trunks found 

 huddled together all over the area are greatly in excess 

 of the number of trees that could have grown to a 

 vigorous maturity on such a limited surface. That the 

 action of silicificatiqn set in after the destruction of the 

 forest is a matter of universal acceptance; yet a very 

 interesting theory could be developed on this point from 

 the existence round the base of the trees of a thick 

 sandstone coating which lessens on, or entirely disap- 

 pears from, the bark encasing the topmost sections. 



In the third and largest section of the forest, thirteen 

 miles from Adamana, there are several liundred whole 

 trunks partly embedded in the earth, some of which 

 exceed one hundred and fifty feet in length. The 

 colours here are very striking, and the crystal deposits 

 of considerable frequency. The second or middle forest 

 is the smallest of the three, but in its two thousand 

 acres are many fine specimens of wood agate, those 

 named the Twin Sisters being the most widely known. 



The distance between Adamana and the first forest 

 is only six miles, and on the road between there are 

 prehistoric ruins made out of logs of this fossil wood, 

 wliich also served for the construction of implements 

 found in Pueblos hundreds of miles away in the desert. 

 The majority of the trees in all parts of the dead forest 

 are broken off in sections, ranging from two to twenty 

 feet, but the first forest, which has a higher altitude, 

 possesses one of the most remarkable exceptions, which 

 imquestionably grew on the spot where it now lies. It 

 measures one hundred and eleven feet in length, with 

 a base diameter of four feet four inches, graduating to 

 eighteen inches at the other end, and in its prostrate 

 position it spans a fifty feet wide canyon, making a 

 unique natural bridge over the intermittent river, whose 

 bed is twenty feet from the canyon's edge. 



^^^^^^ 

 An Interesting New Asteroid. 



On February 22, if)i)(), a new astemid was detected by 

 Max Wolf at Heidelberg, which was found to have a re- 

 niark.ibly siniill rate of retrogradation, about 30 sees, daily. 

 The provisional designation of this body is TO. Later Dr. 

 Herbcrich deduced circular elements for the asteroid, which 

 indicated that its mean distance was S-OS- almost identical 

 with that of Jupiter, and from more recent observations, 

 April 22, combined with those of February 22 and March 23, 

 has computed elliptic elements. These show a mean 

 distance a little greater than Jupiter, and aphelion distance 

 al)()iit one unit beyond the orbit of Jupiter. A. C. D. 

 Cninmielin dr.aws attention to (he fact that Ibe family of 

 asteroids now extends from distance i.i (perihelion of Eros) 

 to distance 6 (aphelion of TG), and in consequence the in- 

 vestigation of the perturbations of T(i by Jupiter should 

 prove very interesting on account of the equality of their 

 mean motions. In the present positions of the orbits no 

 very close approach occurs. — (Observatory, June, 1906.) 



"The value of glass may far exceed that of gold," says 

 Amateur Wurk (Boston). " A contemporary draws atten- 

 tion to its enormously increased value when made up into 

 microscope objectives. The front lens of a micro-objective, 

 costing 5 dollars, does not weigh more than about 0.00 iS 

 gram, which weight of gold is worth about one cent, and 

 so (he value of a kilogram of such lenses would be about 

 J,ooo.ooo dollars. The cost of the raw material for making 

 this weight of glass is from 5 cents, and thus, when 

 wvirUrd up into the shape of a lens, the glass has been 

 iiii iia^cd in value aliout fifty million times. Such disparitv 

 brlwi'cn the cost of the raw material and the manufactured 

 article is probably a record in industrial technics." 



Photography. 



Pure and Applied. 



By Chapman Jones, F.I.C, F.C.S., >.\:c. 



T/ie Efficioicy of Shui/crs. — Although shutters are 

 now very commonly employed, there are many false 

 ideas held with regard to them. It has been stated so 

 frequently that it seems to be commonly accepted as 

 a fact, that focal-plane shutters gi\e an efficiency of 

 one hundred per cent. Before making any precise 

 statements, it is necessary to make clear what is meant 

 by the word cfjiciency. If it is defined as the ratio 

 between the actual period of the exposure and the time 

 that would be necessary to produce the same light 

 effect if the shutter were removed, that is, if the open- 

 ing and closing took no time at all, then the focal-plane 

 shutter is the least efficient of any pattern, for the time 

 taken for tlie narrow slit to pass across the front of 

 the plate is always considerable, because of the great 

 distance it has to travel. But if efficiency is taken as 

 applying, not to the whole plate, but to each point of 

 it separately, so giving the focal-plane shutter all 

 possible advantage, still its efficiency is not, and never 

 can be, one hundred per cent. Considering shutters 

 of ail classes so far as I know them, the general state- 

 ment, that in giving their minimum exposures the 

 efficiency tends towards fifty per cent, may be accepted 

 as correct. That is, the actual period is about twice 

 as long as would give the same light effect if the 

 uncovering and r€-covering of the plate or lens t(X)k 

 no timeatall. Theapplicationof this pruicipleto the focal- 

 plane shutter is not generally appreciated. By reason of 

 the exigencies of construction the slotted blind must bean 

 appreciable distance in front of the sensitive surface, 

 and the aperture of the lens must be considerable. 

 It therefore follows that each edge of the opening gives 

 a penumbra, and that the exposure of e\ery point of the 

 plate begins gradually, increases to a maximum, and 

 then diminishes to zero, as in all other shutters. There 

 are some shutters that give an efficiency of less than 

 fifty per cent., but this is because either of faulty con- 

 struction or the endeavour to get an exposure that is 

 shorter than tlie apparatus will properly give. The 

 advantage of the focal-plane shutter is not in its greater 

 efficiency when giving its minimum exposure, but that 

 its shortest exposure (having regard to only one point 

 of the plate) is, say, about a tenth of the shortest ex- 

 posure given by a lens-shutter, so that in giving the 

 same exposure with both of, say, the one-hundredth 

 of a second, the lens-shutter is working at its quickest 

 and with its minimum efficiency, while the focal-plane 

 shutter is giving ten times its shortest exposure with a 

 corresponding gain in efficiency. On the other hand, a 

 diaphragm shutter that opens from the centre will 

 permit of the use of a larger opening in the lens for 

 the same defining power and depth of definition, be- 

 cause the greater part of the exposure is given \\\\\\ 

 smaller apertures than the full, by reason of the shutter 

 acting the part of an expanding and contracting 

 diaphragm. There are many other details that might 

 be referred to in this connection, for the matter is much 

 more complex than it is generally considered to be. 



What is the Onc-hundrcdth of a Second?. — It is eas>- to 

 talk glibly of such an exposure as this, and to engrave 

 the fraction on the shutter, but the definition of it is 

 not so simple. Of course, such a period of time ? 



