March 6, 1902] 



NA TURE 



429 



In 1895 Mr. Lanchester pointed out to the author that such 

 an enclosure would be a theoretically perfect radiator ; while 

 Lumtner, Paschen and others using radiation from such a 

 source have confirmed in a remarkable manner Stefan's law of 

 radiation, viz. R = liT*. 



The radiator employed was a porcelain ' tube 2 feet long and 

 I inch internal diameter fitted into a Fletcher gas-tube furnace. 

 A plug of asbestos was inserted in the tube about lo inches 

 from the end remote from the radio-micrometer, and resting 

 against this plug was the end of a Callendar platinum resistance 

 thermometer. In front of the open end of the tube was a 

 rectangular aperture 5 mm. wide in a large brass water-screen : 

 a slide closing this aperture was moved by a micrometer screw 

 reading to O'oi mm. This aperture 

 was 66'3 mm. from the surface of the 

 thermocouple (Fig. l). 



To make an observation, the tube 

 was heated to as high a temperature 

 as the furnace was capable of, and the 

 radiation from the interior of the tube 

 passing into the aperture (p.) of the 

 radio-micrometer w.as adjusted by the 

 micrometer screw until a balance wa^ 

 obtained with the radiation of the sun 

 through the aperture (A). 



After a series of observations ha>l 

 been made, the arrangement was altereil 

 so that the radiation from the tuht- 

 should enter aperture (.\) and from the 

 sun aperture (b) of the radio-micro 

 meter, and in this position a second 

 series of observations was taken. Tli.- 

 geometric mean of the results of the tw^ i 

 groups gives the effective temperature 

 of the sun. 



The mean of the observations thus 

 made gave 5773° C. (absolute) as the 

 sun's effective temperature. 



In calculating this result, Rosetti's 

 coefficient of atmospheric absorption, 

 viz. o'29, has been used. Takini; 

 Langley's value, viz. o'4i, the result 

 will be 6085° C. (absolute). 



It is interesting to allow for the 

 eff'ecl of absorption in the sun's atmo- 

 sphere. Assuming the results of 

 Wilson and Rambaut's experiments 

 (Proceedings Royal Irish Academy, 

 1S92, vol. ii. No. 2), the value 6863° C. 

 (absolute) is deduced as the effective 

 temperature of the sun's photosphere. 



Physical Society, February 28.— 

 Prof. S. P. Thompson, president, in 

 the chair. — A paper on focal lines and 

 anchor-ring wave-fronts was read by 

 Prof. J. D. Everett. When a small 

 cone of rays is obliquely incident on a 

 spherical reflecting or refracting surface, 

 the rays after reflection or refraction no 

 longer compose a true cone. Instead of 

 meeting in a point they form a narrow 



neck, and this neck is flattened in two Fi 



places called the two focal points, the 

 planes of flattening being at right angles 



to each other. Optical writers give the name focal Urns to 

 the sections of the pencil made at the focal points by planes 

 perpendicular to the axis of the pencil ; but it would be 

 more appropriate to give the name to the sections which most 

 nearly resemble lines, whatever angle they make with the axis 

 of the pencil. Attention is drawn in the present paper to the 

 case in which the wave-front in one of its positions is a tore (or 

 anchor-ring). Even when dealing with wide-angled pencils 

 there are then two well-defined focal lines, the primary focal 

 line being what may be called the circular axis of the tore, and 

 the secondary a portion of the line about which the generating 

 circle turns to form the anchor-ring. Toric wave-fronts can be 

 produced by reflection from a mirror made by allowing an 

 ellipse or portion of an ellipse to revolve completely round 



1 In later experiments an iron tube was substituted. 



NO. 1688, VOL. 65] 



an ordinate erected at one focus, and employing it to reflect 

 rays diverging from a small source at the other focus. The 

 primary line is always real ; the secondary is real or virtual 

 according to the position of the area of incidence of the 

 pencil. — A paper entitled "Contributions to the Theory of 

 the Resolvrng Power of Objectives" was read by Prof. 

 Everett. The practical limit to the resolving power of ob- 

 jectives depends upon the blurring due to diffraction. Obser- 

 vations on double stars for the purpose of investigating the 

 separating power of telescopes have been made by Dawes, who 

 arrived at the conclusion that the angular distance between the 

 two components, when they are nearly equal in magnitude and 

 are just separated, is given by the formula, 4-56 seconds 



-DitTerential radio-micrometer with tube furnace. 



divided by the diameter of the objective in inches. Foucauh 

 also investigated the matter experimentally, and in 1830 Airy 

 calculated the brightness at various points of the spot and rings 

 which constitute the im.ige of a point source formed by an 

 objective. If the extreme difference of optical path for dis- 

 turbances coming from different points of a concave wave-front 

 to a point at lateral distance b from the geometric focus is made 

 equal to the wave-length of light, a value for h is obtained which 

 represents with fair accuracy the limit of separation as deter- 

 mined by experiment. The formula agrees with that of Dawes 

 if X = -56 micron., whereas the wave-length of the brightest ray 

 is usually taken as -55 micron. In the case of microscopes the 

 author has supposed that the formula for the minimum distance 

 b still holds good, and combining this equation with the sine 

 condition applicable to optical systems giving sharp flat images, 

 he has deduced the expression which has been extensively used 



