February 15, 1894] 



NA TORE 



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certainty of the method. — A compensating open-scale barometer 

 was then exhibited and described by Mr. Griffiths. The 

 principle of this instrument is the same as that of Prof. 

 Callendar's long distance air thermometer. An air bulb is 

 placed within a second bulb, and the annular space between 

 them is filled with sulphuric acid. The air and the H0SO4 

 have a common surface in a tube connecting the two bulbs, the 

 HoSOj also communicates with the air by means of a 

 vertical tube partially lilled with acid. The masses 

 of air and sulphuric acid are so adjusted that when the tempera- 

 ture of the instrument is raised, the increase in pressure due to 

 the increased length of the sulphuric acid column in the vertical 

 tube exactly counterbalances the increase in pressure of the con- 

 tained air, and thus the position of the common surface is un- 

 changed by alterations in temperature, although at once affected 

 by alterations in the external pressure. The resulting scale is 

 about six times as open as the scale of a mercury barometer, and 

 the readings give the pressure expressed in terms of the length 

 of a column of mercury at 0° C. in latitude 45°, without any pre- 

 liminary calculations. — On the condition of the interior of the 

 earth, by Rev. O. Fisher. The author has lately calculated the 

 tidal deformation of a liquid earth owing to the attraction of the 

 moon, assuming Laplace's law of density ; the moon's potential 

 is substituted for that of the centrifugal force in the usual calcu- 

 lation of the earth's figure by means of Laplace's functions ; 

 and the result obtained is a deformation of 3"45 feet, or 690 

 feet from highest to lowest. This value is nearly four times as 

 great as a value used in an earlier paper " On the hypothesis of 

 a liquid condition of the earth's interior, &c." read in May, 

 1892. The calculation of the new value leads the author to 

 consider that the first three pages of the earlier paper lose their 

 force, though the remaining portions stand unaffected. The 

 author points out that the existence of ocean tides is not a con- 

 clusive argument in favour of rigidity, inasmuch as on the hypo- 

 thesis of liquidity mountains must have "roots," sinking deep 

 into the heavier liquid, the result being a deflection of the tidal 

 wave in the substratum, whence would arise irregularities 

 analogous with " establishment of ports." — On a combination of 

 prisms for a stellar spectroscope, by Mr. H. F. Newall. An 

 isosceles and nearly equiangular prism is polished on three faces, 

 and light from a collimator after falling on the base and emerg- 

 ing from one side falls normally on the hypotenusal face of a 

 right-angled prism, and after two reflections within the prism is 

 made to fall upon the third face of the first prism and to emerge 

 from its base. The spectroscope has therefore a dispersion equal 

 to that of two prisms, and is arranged so that the light reflected 

 from the base at primary incidence passes into the same telescope 

 as is used to view the spectrum, and gives rise to a simple image 

 of the slit, which can be used as a luminous pointer. For as- 

 tronomical purposes it is convenient ; for, when the slit is 

 widened, an image of the star can be seen, and the star may be 

 identified among-t its neighbours. The brightness of the pointer 

 is proportioned to the spectrum to be observed. No double ad- 

 justment is necessary in directing the telescope. 



Dublin. 

 Royal Dublin Society, December 20, 1893. — Prof. D. J. 

 Cunningham, F.R.S., in the chair. — Dr. G. Johnstone Stoney 

 read a paper upon vision, with special reference to vision with 

 compound eyes. The most interesting points brought out by 

 this investigation are the two following: — (i) The amount of 

 detail that is visible by human beings is limited by the spacing 

 of the cones in the macula lutea of the human eye, by the 

 limited size of the pupil, and by spherical and chromatic 

 defects in the eye regarded as an optical instrument. In 

 persons w?th the best vision, these three limiting causes concur 

 in fixing about one minute of arc as the smallest angular in 

 terval to be subtended by two objects at the eye, in order that 

 they may be visible as two. With an insect's compound eye 

 a corresponding limit is placed by the spacing of the lenses 

 over its cornea, and by the small aperture of each lens. Judg- 

 ing from these, we learn that predatory insects, such as dragon- 

 flies, which have the largest number of lenses, see so much less 

 perfectly than we do that the angular interval at which two 

 objects must stand to be seen as two, is nearly a degree ; while 

 in moths, butterflies, bees, ordinary flies, &c. which have not 

 this great number of facets, the angular interval that is requi- 

 site rises to be two degrees or more : so that such insects do 

 not see details upon their own antennas, close to them as they 

 are, so distinctly as we can see them from the great distance 

 from which we are obliged to view them. Moreover, when 



NO 1268, VOL. 49] 



the number of facets ha? to be increased, as it is in predatory 

 insects, in order to improve their vision, it is necessary at the 

 same time that the aperture of each lens should not be unduly 

 diminished. This accounts for why the compound eyes of 

 such insects are of excessive size when compared with their 

 other features. (2) Again, our eyes see distinctly only a small 

 central patch of the field of vision, but can be directed 

 towards various objects in succession by rotating the eye in its 

 orbit, and can be accommodated to the distance of each. There 

 is no such motion of rotation possible to insects, but in com- 

 pensation they seem to be able to see distinctly throughout the 

 whole of the field of vision, and to have the remarkable power 

 of being able simultaneously to adjust the different parts of 

 their compound eye to see distinctly at different distances, so 

 that, for instance, a wasp hovering over a break''a5t-table can 

 accommodate his eyes to see with as much distinctness as the 

 insect can see, the several objects on the table, though they 

 may be at very different distances from him. — Dr. J. Joly, 

 F.R.S., read a paper on the effect of temperature upon the 

 sensitivness of the photographic dry plate, of which the follow- 

 ing is a brief abstract : The visible spectrum photographed 

 upon plates one-half of which were maintained at a lov/ tem- 

 perature (about - 30° C. ) and the other half kept warm, showed 

 that the loss of sensitiveness is in the case of isochromatic plates 

 confined almost entirely to the yellow-green and green-blue. In 

 fact the sensitiveness ordinarily conferred by the action of the 

 dye is annulled save for some survival of the very strong band 

 in the green, which is continued, much weakened, from the 

 warm half across the cold half, and without shift. It appears 

 from this that the use of orthochromatic plates in cold climate-; 

 out of doors offers little or no advantage over ordinary gelatino- 

 bromide plates. The spectrum taken upon a cold region on 

 the ordinary gelatino-bromtde plate shows a very slight weaken- 

 ing throughout, but most markedly in the rays of lowest 

 refrangibility. The feeble action of the dye at low tempera- 

 tures seems to confirm Abney's view that the action of the dye 

 is mainly of a chemical nature. — At the meeting held January 

 17, Prof. J. Mallet Purser in the chair, the following 

 communications were presented : — Dr. J. Joly, F. R. S., demon- 

 strated some simple methods in teaching elementary physics. 

 By the use of a floating piston (a contrivance enabling a wide 

 column of mercury to be supported without friction or risk of 

 falling out in a tube), the author uses as a "Boyle's tube'' a 

 uniform straight tube about i metre long, closed at one end. 

 The tube is placed vertical with the closed end downwards, a 

 certain volume of air v^ (defined by linear measurement upon 

 the tube) is enclosed by a short column of mercury ; the length 

 of this added to the height of the barometer affords P^. The 

 air is now further loaded with mercury ; and y, and P., 

 measured as before. The operations are evident at a glance, 

 and very accurate results may be obtained. To show the lae 

 of thermal expansion of air and to convey the meaning of absolute 

 zero, by gas therometer, the end of the lube — all as above — is 

 placed in melting ice, and mercury added till the air occupies 

 273 mm. of the tube. It is then dipped into a flask of boiling 

 water having a long neck. The column of air now increases 

 to 373 mm. when the usual inferences may be drawn. — Prof. 

 D. J. Cunningham then gave a magic-lantern demonstration 

 of the development of the convolutions and fissures of the 

 human brain 



Paris. 



Academy ot Sciences, February 5. — M. Lcewy in the 

 chair. — On the propagation of sound against various resistances 

 in a fluid, by M. J. Boussinesq. An analytical determination of 

 the problem discussed in several recent coamiunications. — On 

 the propagation of electromagnetic waves, by M. Mascart. The 

 mean speed of propagation is given as 302,850 km. rejecting the 

 more doubtful results. No regular variation with the length of 

 the waves is apparent. — On the theory of the satellites of Jupi- 

 ter, by M. J. J Landerer. — On the temperature of the higher 

 regions of the atmosphere, by M Alfred Angot. A reply to a 

 recent criticism by M. G. Hermite. — On the thermal value of 

 the replacement of phenolic hydrogen in orcin, by M. de For- 

 crand. The heat of solution for i mol. of anhydrous orcin in 

 2 litres of water at 10° C. is - 2 64 Cal. The mean value for re- 

 placement of one atom of hydrogen by one atom of sodium is 

 + 39'68 Cal., a number very near those given by other plenols. 

 — On campholene, by M. Guerbet. The author obtains a 73 

 percent, yield by distilling CiqHj-C10 in presence of a trace 

 of phosphoric anhydride. Campholene yields a a hydrocarbon 



