iS8 



NA TURE 



[June 23, 1892 



in order to make a start we must assume that the value of i is 

 known for some definite value of/. It is not of much consequence 

 what assumption, within reason, is made, as, though the cal- 

 culated curves wiU vary with the assumption made, they will all 

 eventually merge into the true periodic current curve at some 

 point which will be exhibited when the first evanescence of 

 A^-RC'^L takes place. 



As this complementary function is a continually decreasing 

 quantity, it becomes negligible when it is allowed time enough. 

 This opportunity is afforded when the straighter portions of the 

 (B, H) curve are reached. 



In the paper are given diagrams showing the plotted curves 

 of current calculated from the above equations, together with 

 tables of calculated values. 



"The Conditions of the Formation and Decomposition of 

 Nitrous Acid." By V. H. Veley. 



The main points of this inquiry may briefly be summarized as 

 follows : — 



(i) The formation of the impurity of nitrogen peroxide in 

 nitric acid, imparting to it the veil-known yellow tint, takes 

 place in the case of the more concentrated acid, even at a tem- 

 perature of 30°, and of the less concentrated acids at from 

 loo°-i5o°, even when the acid is not unduly exposed to sunlight. 



(2) The reaction between nitric oxide and nitric acid may be 

 regarded as reversible, thus : — 



2NO -f HNO3 -f H2O ^ 3HNO2, 



provided that the acid be sufficiently dilute, and the temperature 

 sufficiently low. Under these conditions equilibrium is estab- 

 lished between the masses of nitric acids when the ratio of the 

 former to the latter is, roughly speaking, as 9 : i. The actual 

 ratio varies slightly on the one side or the other, according to 

 the conditions of the experiments. With more concentrated 

 acids and at higher temperatures the chemical changes taking 

 place are more complicated, and the decomposition of the acid 

 more profound. 



(3) The decomposition of solutions containing both nitric 

 and nitrous acids is also investigated ; the rate of the change is 

 shown to be proportional to the mass of the nitrous acid under- 

 going change. The curve representing the amount of chemical 

 decomposition in terms of the mass present is shown to be hyper- 

 bolic, and illustrative of the law 



(I. 



This holds good, whatever be the method employed for the pro- 

 duction of the nitrous-nitric acid solution. 



The observed values for C or the concentration of the nitrous 

 acid are concordant with those calculated according to the 

 above differential equation within the limits of experimental 

 error. 



The rate is dependent upon the ratio of the masses of the 

 nitrous and nitric acid, being the more rapid, the greater the 

 proportion of the former to that of the latter. 



In the particular case of the liquid prepared from nitric oxide 

 and nitric acid, wherein the reproduction of solutions of similar 

 concentration presents less difficulty, it is &h )wn that as the 

 temperature increases in arithmetical the rate of change in- 

 creases in geometrical proportion, in accordance with the 

 equation 



(II.) Vt = Vt,B'-\\ 



the value for k being 0-0158. 



Finally, though the niirous-nitric acid solutions behave in a 

 similar manner as regards the diminution of the mass of the 

 nitrous acid, yet, in other respects, such as evolution of gases 

 and the action upon metals, they are dissimilar, 



" On the Method of Examination of Photographic Objectives 

 at the Kew Observatory." By Major L. Darwm. Communi- 

 cated by Captain Abney, R.E., F.K.S. 



The paper describes the method of examination of photo- 

 graphic objectives which has been adopted at the Kew Observa- 

 tory, chiefly on the recommendation of the author. In selecting 

 and devising the different tests, Major Darwin acted in co-opera- 

 tion with Mr. Whipple, the Superintendent of the Ojservatory, 

 and was aided by consultations with Captain Abney. 



The object of the examination is to enable any one, on the 

 payment of a small fee, to obtain an authoritative statement or 

 certificate as to the quality of an objective for ordinary purposes. 



NO. 1182, VOL. 46] 



An example is first given of a "Certificate of Examination," 

 such as would be obtained from Kew, and then the different 

 tests are discussed in detail. The following are the different 

 items in the Certificate of Examination, or the various tests to 

 which the objective is subjected : — 



(I) to (4) None of this information forms part of the result 

 of the testing. 



(5) Number of External Reflecting Surfaces. 



(6) Centering in Mount. 



(7) Visible Defects, such as Veins, Feathers, &^c. 



(8) Flare Spot. 



(9) Effective Aperture of Slops, which is given for each one 

 supplied with the objective. In recording the results, it is 

 proposed that the system of numbering recommended by the 

 International Photographic Congress of Paris of 1889 should be 

 adopted. 



(10) Angle of Cone of Illumination, Ssr'c. 



(II) Principal Focal Length. — This is found by revolving 

 the camera through a known angle, and measuring the move- 

 ment of the image of a distant object on the ground glass ; 

 with the Testing Camera it is so arranged that an angular 

 movement can be given with great ease and accuracy, and that 

 the angle is such that half the focal length is directly read off 

 on a scale on the ground glass. The observation is made when 

 the image is at a point some 14 degrees from the axis of the 

 objective, and the effect of distortion and curvature of the field 

 is discussed ; it is proved that the focal length thus obtained, 

 even though it may not be identical with the principal focal 

 length as measured on the axis, is nevertheless what the photo- 

 grapher in reality wants to ascertain. 



( 1 2) Cw-vature of the Field. 



(13) Distortion. — This test depends in principle on ascertain- 

 ing the sagiita or deflection in the image of a straight line 

 along one side of the plate. In the discussion it is shown that 

 to give the total distortion near the edge of the plate would not 

 answer practical requirements, and that the proposed method 

 of examination does give the most useful information that can 

 be supplied. 



(14) Definition. — This is found by ascertaining what is the 

 thinnest black line the image of which is just visible when seen 

 against a bright background. It is shown that this is the best 

 method that could be devised of measuring the defining power 

 of an objective, and that it is not open to serious objections on 

 theoretical grounds. 



(15) Achromatistn. — In the Certificate is recorded under this 

 heading the difference of focus between an object when seen 

 in white light and the same when seen in blue or red light. 



(16) Astigmatism. — This test is performed by measuring the 

 distance between the focal lines at a position equivalent to the 

 corner of the plate, and by calculating from the result thus 

 obtained the approximate diameter of the disk of diffusion due 

 to astigmatism. 



(17) Illumination of the Field. — The method of examination, 

 which is due to Captain Abney, is described. 



"On Certain Ternary Alloys. Part VI. Alloys containing 

 Aluminium, together with Lead (or Bismuth) and Tin (or 

 Silver)." By C. R. Alder Wright, D.Sc, F.R.S., Lecturer on 

 Chemistry and Physics in St. Mary's Hospital Medical School. 



The experiments described in this paper are a continuation 

 of the previous researches on the miscibility of molten 

 metals under such conditions that whereas two of the 

 metals, A and B, will not mix together in all propor- 

 tions, ahe third, C, is miscible in all proportions with either 

 A or B severally. The alloys now investigated are those where 

 A is lead (or bismuth) ; B, aluminium ; and C, tin (or silver). 

 They show considerable analogy with, and resemblance to, those 

 previously described containing the same metals as A and C re- 

 spectively, but zinc instead of aluminium as B ; but certain 

 differences are noticeable : thus the substitution of aluminium for 

 zinc invariably raises the critical curve, causing it to lie outside 

 its former position, this being observed whether the heavy 

 immiscible metal, A, be lead or bismuth, and whether the solvent 

 metal, C, be tin or silver. On the other hand, the substitution of 

 bismuth for lead always depresses the critical curve, causing it to 

 lie inside its former position ; this being equally observed 

 whether the lighter immiscible metal, B, be zinc or aluminium, 

 and whether the solvent metal, C, be tin or silver. In the case 

 of the metals bismuth-zinc-silver and lead-zinc-silver, peculiar 

 bulges (inwards and outwards) were noticed in ceitain parts of 



