214 



NA TV RE 



[June 29, 1893 



upon the surface, the rule is that thebcinds, or any parts of them, 

 i^ay be rubbed in the direction of the arrow. 

 " A good many surfaces have thus been operated upon ; and 

 although a fair amount of success has been attained, further 

 experiment is required in order to determine the best procedure. 

 There is a tendency to leave the marginal parts behind ; so that 

 the bands, though straight over the greater part of their length, 

 remain curved at their extremities. In some cases hydrofluoric 

 acid has been resorted to, but it appears to be rather difficult to 

 control. 



The delicacy of the test is sufficient for every optical purpose. 

 A deviation from straightness amounting to yV of ^ band inter- 

 val could hardly escape the eye, even on simple inspection. 

 This corresponds to a departure from flatness of tjV of a wave- 

 length in water, or about ^ of the wave-length in air. Probably 

 a deviation of y|j \ could be made apparent. 



For practical purposes a layer of moderate thickness, adjusted 

 so that the two systems of bands corresponding to the duplicity 

 of the soda line do not interfere, is the most suitable. But if 

 we wish to observe bands of high interference, not only must 

 the thickness be increased, but certain precautions become 

 necessary. For instance, the influence of obliquity must be 

 considered. If this element were absolutely constant, it would 

 entail no ill effect. But in consequence of the finite diameter 

 of the pupil of the eye, various obliquities are mixed up together, 

 even if attention be confined to one part of the field. When the 

 thickness of the layer is increased, it becomes necessary to reduce 

 the obliquity to a minimum, and further to diminish the aperture 

 of the eye by the interposition of a suitable slit. The effect of 

 obliquily is shown by the formula 



2 / (i - cos 9) = « A. 



The necessary parallelism of the operative surfaces may be 

 obtained, as in the above-described apparatus, by the aid of 

 levelling. But a much simpler device may be employed, by 

 which the experimental difficulties are greatly reduced. If we 

 superpose a layer of water upon a surface of mercury, the flatness 

 and parallelism of the surfaces take care of themselves. The 

 objection that the two surfaces would reflect very unequally may 

 be obviated by the addition of so much dissolved colouring 

 matter, e.g. soluble aniline blue, to the water as shall equalise 

 the intensities of the two reflected lights. If the adjustments 

 are properly made, the whole field, with the exception of a 

 margin near the sides of the containing vessel, may be brought 

 to one degree of brightness, being, in fact, all included within a 

 fraction of a band. The widih of the margin, within which 

 rings appear, is about one inch, in agreement with calculation 

 founded upon the known values of the capillary constants. 

 During the establishment of equilibrium after a disturbance, 

 bands are seen due to variable thickness, and when the layer is 

 thin, persist for a considerable time. 



When the thickness of the layer is increased beyond a certain 

 point, the difficulty above discussed, depending upon obliquity, 

 becomes excessive, and it is advisable to change the manner of 

 observation to that adopted by Michelson. In this case the eye 

 is focused, not, as before, upon the operative surfaces, but upon 

 the flanr.e, or rather upon its image at E (Fig. 2). For this pur- 

 pose it is only necessary to introduce an eye-piece of low power, 

 which with the lens C (in its second operation) may be regarded 

 as a telescope. The bands now seen depend entirely upon 

 obliquily according to the formula above written, and therefore 

 take the form of circular arcs. Since the thickness of the layer 

 is absolutely constant, there is nothing to interfere with the 

 perfection of the bands except want of homogeneity in the 

 light. 



But, as Fizeau found many years ago, the latterdifficnlty soon 

 becomes serious. At a very moderate thickness it becomes 

 necessary to reduce the supply of soda, and even with a very 

 feeble flame a limit is soon reached. When the thickness was 

 pushed as far as possible, the retardation, calculated from the 

 volume of liquid and the diameter of the vessel, was found to 

 be 50,000 wave lengths, almost exactly the limit fixed by 

 Fizeau. 



To carry the experiment further requires still more homo- 

 geneous sources of light. It is well known that Michelson has 

 recently observed interference with retardations previously un- 

 heard of, and with the aid of an instrument of ingenious con- 

 struction has obtained most interesting information with respect 

 to the structure of various spectral lines. 



A curious observation respecting the action, of hydrofluoric 

 NO. 1235, VOL. 48] 



acid upon polished glass surfaces was mentioned in conclusion. 

 After the operation of the acid the surfaces appear to be covered 

 with fine scratches, in a manner which at first suggested the idea 

 that the glass had been left in a specially tender condition, and 

 had become scratched during the subsequent wiping. But it soon 

 appeared that the effect was a development of scratches pre- 

 viously existent in a latent state. Thus parallel lines ruled with 

 a knife edge, at first invisible even in a favourable light, became 

 conspicuous after treatment with acid. Perhaps the simplest 

 way of regarding the matter is to consider the case of a furrow 

 with perpendicular sides and a flat bottom. If the acid maybe 

 supposed to eat in equally in all directions, the effect will be 

 to broaden the furrow, while the depth remains unaltered. It is 

 possible that this method might be employed with advantage to 

 intensify (if a photographic term may be permitted) gratings 

 ruled upon glass for the formation of spectra. 



FROST FREAKS. 



lyrR. LESTER F. AVARD describes some remarkable frost 

 figures in the current number of The Botanical Gazette. He 

 says that on a bright frosty morning inDecember, 1892, Mr. Victor 

 Mason and himself observed some white objects looking like 

 icicles close to the ground, along the border of a pine wood. 

 A closer exaujination showed that they were in truth nothing 

 but ice, but that instead of icicles they were veritable freaks 

 of frost. Every one was firmly attached to the stem of a 

 small herbaceous plant which had succumbed to the season but 

 still stood erect. The attachment was always close to the base, 

 often at the very ground, sometimes an inch above. At a dis- 

 tance, the frost-works had the appearance of cylindrical masses, 

 but one need not come very near to see that such was not the 

 case. In fact, they really consisted of several thin foils or wings 

 from one to three inches in width, firmly attached by one edge 

 to the stem of the plant, thus standing in a vertical position. 



From this attachment each of these little ice sheets projected 

 out horizontally or with a slight upward tendency, not straight 

 and stiff, but gently and gracefully curving or coiling into a 

 beautiful conch-like roll at the distal margin. There were 

 always several of these, usually three, four, or five, all attached 

 to the same vertical portion of the stem but at regular intervals 

 around it like the paddles of a flutter-wheel, but all curving in 

 the same direction after the manner of a turbine-wheel. Thus, 

 where there were four they stood with each pair opposite, as in 

 the figure, which represents a side view. The amount of curving 

 varied considerably, and the coil filled up most of the intervd 

 between the plates giving the object a compact appearance. 

 The ice was white, opaque, and singularly light, as if consisting, 

 of congealed froth, but in all cases the scrolls bore horizontJU 

 stripes like those of a flag, resulting from degrees in the white-. 

 ness, varying from alabaster to neariy transparent. These stripes 

 added greatly to the beauty of these singular objects. In some 

 cases the inner margin, instead of being straight, was sinuous, 



