lyo 



NA rURE 



[June 22, 1893 



Rogers. — Note on some properties of Gauche cubics, by Mr. 

 T. R. Lee. There are two principal theorems in the note, one 

 being an analogue of the theorem of Desargues, and the other 

 affording a test by which it may be determined whether a given 

 line is a chord of a cubic or not. — Note on the centres of simili- 

 tude of a triangle of constant form circumscribed to a given 

 triangle, by Mr. J. Griffiths. 



Physical Society, June 9. — Prof. J. Perry, F.R.S., 

 Vice-President, in the chair. — Mr. A. P. Trotter read a 

 paper on a new photometer. The author has modified 

 his illumination photometer, described Proc. I.C.E., vol. 

 ex. paper No. 2619, so as to adapt it to the measure- 

 ment of candle-power. The principle employed is to view 

 a screen illuminated by one source through an aperture in 

 a second screen illuminated by the other light, the aperture be- 

 coming invisible when the illuminations are equal. After using 

 perforations of various patterns, a series of narrow slits cut in 

 thin paper were found to give the best result. The plain screen 

 is mounted behind the slotted one in a box sliding on the 

 photometer bench, and they are arranged so that the light falls 

 on them at equal angles. The screens are viewed from a dis- 

 tance of 6 or 7 feet through an opening in the front of the 

 box, cords being provided for producing the traversing 

 motion. Two "sights" set respectively at the middle of the 

 length of the plain screen, and on the lower edge of the front 

 opening, serve to show when the middle of the band of equal 

 illumination is vertically above the pointer on the carriage. 

 The photometer is found to be particularly valuable when it is 

 desired to determine the maximum power of a variable source. 

 When lights of different colour are being compared — say a gas 

 flame and an arc — one end of the screen shows blue strips on 

 a yellow ground, and the other end yellow strips on a blue 

 ground ; at the centre the colours seem to blend. Te facilitate 

 the comparison of such lights, Mr. Crompton, who has been 

 working at the subject simultaneously with the author, uses one 

 screen tinted pale yellow and the other pale blue. Details of 

 construction of the new photometer are given in the paper, and 

 the accuracy attainable when comparing two equal lights of 

 about eight candles, stated to be about I per cent. — Prof. S. P. 

 Thompson, F.R. S., read some notes on photometry. The first 

 note relates to the use of two overlapping screens as an iso- 

 photal, and describes the evolution of the Thompson-Starling 

 photometer. In this instrument a prismatic block with apex 

 upwards rests crosswise on the photometer bench, and the in- 

 clined sides are respectively illuminated by the two sources to 

 be compared. In testing differently-coloured lights, coloured 

 stuffs were placed over the surfaces of the wedge. In some 

 cases notched and overlapping cards were used to form the 

 overlapping surfaces ; an inclination of about 70" between the 

 two surfaces was found convenient. The second note refers to 

 the periodic principle in photometry, and in it the author dis- 

 cusses the various methods which have been, or may be, used 

 for producing small differences of decreasing amount between 

 the two sides of a photometer screen. By employing a device of 

 this kind much greater accuracy of adjustment is possible. In 

 one form of vibration-photometer worked out by the author, 

 the paraffin blocks of a Jolly's photometer are mounted at one 

 end of a spring, the other end being fixed to the carriai;e. The 

 act of moving the carriage starts the blocks vibrating, thus 

 producing the desired variations. In a third note the question 

 of using the electric arc as a standard of light is dealt with. 

 Since 1878 the positive crater has been used as a standard of 

 whiteness, and last year both the author and Mr. Swinburne sug- 

 gested that a given area of crater might be used as a standard of 

 light. This proposal has since been carried out by M, Blondel. 

 Since the intrinsic brilliancy of the crater rs high, it necessi- 

 tates very small apertures, or else the use of standards of 

 large candle-power. Advantages of using powerful standards 

 are pointed out in the paper. With a circular hole i m/m in 

 diameter, a standard of about fifty five candles could be 

 obtained ; with such a source, benches longer than usual would 

 be preferable. At the end of the note, the errors-which may be 

 introduced by using as an arc standard a hole in a plate of 

 sensible thickness, when viewed obliquely, are investigated, as 

 well as those due to inaccuracy of setting the plane of a hole 

 made in foil, perpendicular to the photometer bench. Major- 

 General Festing, in opening the discussion on both papers, said 

 reflection from the sides of the hole in a thick plate would tend 

 to lessen the error calculated by Prof. Thompson. The ordin.iry 



NO. 1234, VOL. 48] 



impurities in carbon were not likely to alter the brilliancy of the 

 crater. Capt. Abney and himself had no reason to distrust its 

 constancy. Both the vibrating photometer and Mr. Trotter's 

 arrangement would be very useful. — Dr. Sumpnersaid his photo- 

 metric experience had been obtained with the Bunsen, Jolly, 

 and Lummer-Brodhun types. With the two former the in- 

 accuracy arising from uncertainty of adjustment was about \ per 

 cent. Changes of about 0'4 per cent, (average) resulted From 

 reversing the screens. The Lummer-Brodhun instrument (which 

 he described) was better than either of the other two, the aver- 

 age error being about \ per cent. Mr. Frank Wright thought 

 scientific men gave too little attention to the question of light 

 standards. Photometers could be relied on much more than 

 any standard at present in use. The Methven screen was the 

 most practical standard yet devised, but in his opinion no 

 gaseous flame could be a real standard on account of the influ- 

 ence of the surrounding atmosphere. Prof. Ayrton saw diffi- 

 culties in using long benches as suggested by Dr. Thompson, on 

 account of the serious atmospheric absorption which occurs with 

 light from arcs. Decreasing the intensity by dispersion or other- 

 wise was preferable. In some tests on glow lamps now being 

 carried out at the Central Institution, a Bernstein lamp used as 

 a standard was mounted on a spring and vibrated. Mr. Medley 

 showed the vibrating standard referred to by Prof. Ayrton, .ind 

 gave a series of numbers showing that with this device in con- 

 junction with the Lummer-Brodhun photometer accuracies of 

 about i per cent, were obtainable. Mr. Swinburne thought 

 Mr. Trotter's arrangement was better than the "wobbling" 

 photometer. As to the best length of bench, he was inclined 

 to think the shorter the better, provided its dimensions were 

 large compared with those of the standard light. He concurred 

 with Mr. Wright in his remarks about the desirability of obtain- 

 ing a better standard. Speaking of the arc as a standard, he 

 said that only impurities less volatile than carbon would influence 

 the brightness. An important factor was the emmisivity of the 

 carbon, which might not be constant. Mr. Blakesley thought 

 the accuracy obtainable with Mr. Trotter's photometer had been 

 underrated, and pointed out that by using quadrant-shaped 

 screens intersecting orthogonally on the axis of the photometer 

 instead of straight ones, the width of the neutral band could be 

 greatly diminished. Mr. Trotter, referring to Dr. Thompson's 

 paper, said he had found considerable difficulty in making pin- 

 holes suitable for arc standards. It was not an easy matter to 

 accurately measure the hole when made. In photometric 

 measurements he had foimd it very important to reverse his 

 screens. Curved screens as suggested by Mr. Blakesley had 

 been tried, but with little advantage. They also destroyed ihe 

 approximate direct-reading property of the photometer. The 

 subject of changing the length of a bench and its effect on the 

 gradient of illumination was discussed. With short benches 

 one had to guard against the departure from the inverse-square 

 law, due to appreciable size of the standard. Recent experi- 

 ments had shown that the light given out by I square m/m of 

 crater surface differed considerably from 70 candles. — A paper 

 on "The Magnetic Field close to the Surface of a Wire con- 

 veying an Electrical Current," by Prof. G. M. Minchin 

 was taken as read. In this paper the author applies the folution 

 he gave in March last for the conical angle subtended by a circle 

 at any point in space to determine the magnetic potential at a 

 point near the surface of a ring of wire of finite cross section. 

 The shapes of the lines of force near the surface, for several 

 laws of current distribution across the section, have also been 

 worked out. 



Chemical Society, May 18. — Dr. H. E. Armstrong, Pre- 

 sident, in the chair. — The following papers were read : — Studies 

 on the formation of ozone (ii. ), by W. A. Shenstone and M. 

 Priest. Using an ozone generator of the Brodie pattern, the 

 authors have studied the effect of discharges of varying difference 

 of potential upon the quantity of ozone produced. The maximum 

 proportion of ozone that can be produced at a given tempera- 

 ture is nearly independent of the potential diffi--rence employed, 

 provided that this be within 33 and 69 C.G. S. units and that 

 the path of the discharge be not very short at any point in the 

 generator. When this latter case occurs the maximum quantity 

 of ozone that can be obtained has an inverse relation to the 

 difference of potential employed. The rapidity of ozonisation 

 is greater with a high potential difference than with a low one, 

 and the maximum proportion of ozone is produced with a low 

 rate of discharge. A generator made of very thin glass, the 



