July 1,1899.] 



KNOWLEDGE 



1^ 



the photographic plate. A photo of the moving teeth was 

 secured by making one interruption of the primary current 

 in the induction coil, thereby letting one discharge pass. 

 The duration of the radiation was determined by the 

 departm-e from sharpness of outline of the photo ; and he 

 concluded that the duration of a discharge was from -^^^ to 

 ,„^Q Q second. M. Colandeau, adopting the same method, 

 found the result to be about yo\.o second. 



M. Morize adopted Eotti's principle. He fixed the plate 

 to one end of the axle of an electrometer, and at the other 

 end he placed a toothed wheel which interrupts an electric 

 current, and makes a contact which registers itself on a 

 chronograph band. From the broadening of the image 

 of the slit he found the duration of the discharge to be a 

 little over ^-g^i, second. 



Mr. Moffat has now determined that this figure is far 

 too low. In fact he proves that the real duration is about 

 a tenth part of that determined by Colandeau and Morize. 

 He used an optical bank with a screen of barium platino- 

 cyanide and the Eontgen lamp at one end ; an amyl- 

 acetate lamp, as a source of hght, being at the other end to 

 note the standard of comparison. Between these was a 

 movable photometer. In order to obtain as strong Kontgen 

 rays as possible, he introduced an air spark into the • 

 circuit, thereby driving away the particles of dust in the 

 air and making the discharge more sudden. The Eontgen 

 lamp and the spark interrupter were enclosed in a box, the 

 lamp being fixed near one side. At the place where the 

 rays from the anti-cathode were to be transmitted, a small 

 hole was bored and covered with thin black paper. On the 

 other side of this paper the screen was fastened. In front 

 of the screen was fixed a piece of cardboard with a small 

 hole in it, and through this the light-emitting surface of 

 the screen appeared. 



The light from the test lamp had to be weakened, 

 because the light proceeding from the screen was very faint. 

 This was done by putting the lamp Inside a wooden box. 

 A hole was made in front of the box, and covered with 

 translucent paper. In front of this was placed a piece of 

 green glass. For a photometer he used simply two mirrors 

 set at right angles to one another ; in this way very little 

 of the light was lost. To determine the quantity of light 

 emitted by the translucent paper from the amylacetate 

 lamp, he put at one end of the optical bank a lamp, and 

 at the other end the box with the translucent paper thus 

 illuminated. He found that the light emitted by the 

 paper was one-twentieth of that of the lamp. 



He then removed the translucent paper, and determined 

 the extinction co-efficient of the green glass by measuring 

 the intensity of the light transmitted by them in com- 

 parison with that of the lamp alone. He found that the 

 intensity of the light coming from the translucent paper, 

 when weakened by transmission through the green glass, 

 was about - „ Joo lamp. 



Now, the light of the lamp is emitted during the whole 

 time it is burning, but the Eontgen rays only during very 

 short intervals of time. To determine the upper limit to 

 the duration of these intervals, Mr. Moffat put in front of 

 the screen an opaque screen with a small vertical slit, and 

 examined the image of the slit in a rotating mirror. The 

 slit was three millimetres broad, and the mirror was exactly 

 opposite it at a distance of 27'5 cm., revolving at the rate 

 of about ten revolutions per second. Now, he saw that 

 the image was broadened one-tenth part. Accordingly, 

 the duration of a discharge must have been about 



i X 



■^27 5 



44 Ta0005 



second. 



■ By a series of successful experiments Mr. Moffat has 

 averaged the duration of a discharge of the Eontgen rays 

 at rcsoW) second. This, at least, is the upper limit. 



THE STORY OF THE ORCHIDS.-I. 



By the Eev. Alex. S. Wilson, m.a., b.sc. 



TH E family of Orchids is represented in this country 

 by a number of species some of which are among 

 the most handsome of our native wild flowers. 

 Even from a florist's point of view, the purple 

 spikes of 'hrhis mascula and ". lati folia are but 

 little inferior to garden hyacinths and greatly excel them 

 in scientific interest. Some of the tropical kinds, which 

 are so largely cultivated under glass, present the most 

 fantastic shapes and are arrayed in the utmost variety of 

 gorgeous colouring. Orchids are herbaceous plants with 

 sheathing, strap-shaped, parallel- veined leaves ; they have 

 close affinities with the irises, daffodils and lilies. In 

 Monocotyledons the two sets of 

 floral envelopes are coloured alike 

 and not .readily distinguishable ; 

 we do not therefore speak of a 

 calyx and corolla, but of an outer 

 and inner whorl of the perianth. 

 The gay-coloured parts of the 

 orchid are the perianth segments ; 

 Monocotyledons with a bright- 

 hued perianth of this description 

 constitute the sub-clasSj Peta- 

 loidess. As in the irises and 

 daffodils, the ovary of the orchid 

 is placed below the flower ; in the 

 two former it contains three cells ; 

 the ovary of the orchid is one- 

 celled with the ovules attached 

 to the internal walls. During 

 the development of the flower the 

 ovary twists so that those parts 

 which are uppermost in the bud 

 become lowest in the fully ex- 

 panded blossom. The fruit is an 

 elongated capsule which splits by 

 longitudinal slits and discharges 

 the minute seeds. The latter are 

 exceedingly numerous ; the con- 

 tents of a single capsule have 

 been computed in some cases to amount to as many as one 

 million seven hundred and fifty thousand. The name 

 orchis refers to the root, which in the early purple orchis 

 consists of two rounded nodules or tubers. If these be 

 examined after the plant has flowered, one of them is foimd 

 to be soft, flaccid, and withered ; the other hard and fresh. 

 The starchy materials, elaborated by the leaves out of the 

 atmospheric carbonic acid during the summer mouthsj 

 have accumulated in the hard tuber, and in ordinary 

 course would be expended next spring in the production of 

 stem, leaves, and flowers ; the tuber would then have 

 acquired the withered appearance of its neighbour, which 

 has already been exhausted in the formation of the leaves 

 and flowers of the present season. By that time, however, 

 a new tuber would have formed, containing reserve materials 

 for the following year. The tuber is thus a kind of vege- 

 table savings-bank, in which little piles of starch granules 

 take the place of silver and golden coins. The curious 

 bird's-nest orchid receives its name from the rounded mass 

 of interwoven root-fibres. The whole plant is of a dingy 

 brown colour. Having no chlorophyll, it is incapable of 

 assimilating the atmospheric carbon. Its mode of nutrition 

 resembles that of the mushroom, toothwort, and other 

 saprophytes, which obtain their food from decaying organic 

 matter in the soil. 



A large proportion of orchids are epiphytes, or air-plants, 



Fig. 1.— Spike of Early 

 Purple Orchis. 



