The Present Limits of Vision. By Br. Boyston-Figott. 237 
ring, as sharp and true as any draughtsman could draw them with 
indian ink ; but no other rings, either bright or dark, were seen ; 
all were destroyed by the concealing haze. Such fascinating beauty 
of definition in this country is rare indeed. 
I have attempted to represent this effect in Fig. 3, in which the 
dark ring was about the one-fifth of the larger disk. 
By imitating the haze we may thus similarly conceal or destroy 
the outer difi*ractions. Thus the subdued light of the northern or 
north-western sky at sunset, or a green sunset sky in the west, 
portending rain, as people say, may develope marvellous powers of 
definition by reducing the errors of diffraction also. Daylight is 
always preferable to lamplight, and by shading the eye properly, 
much can be then seen which is hidden by artificial glare. I 
believe that many excellent microscopists infinitely prefer day- 
light when sufficiently bright. It is, however, astonishing how 
much the eye gets accustomed to a dim light by promoting the 
expansion of the pupil of the eye. There is no doubt dayhght 
tires less, and therefore injures less in the long run than lamp 
illumination. 
2. The Method of Measuring the Diameter of Solar Disks 
Trigonometrically. 
When rays of light pass through an object-glass centrically 
(as they then pass without deviation), they preserve their original 
direction, and the image and the object both form the same angle 
at the centre of the object-glass. If, therefore, F be the focal 
length, and 32 minutes of arc be the angle subtended by the sun — 
Diameter of sun's image = F sin. 32'. 
And then we have only to multiply the focal length of the lens in 
question by the natural sine of the sun's diameter at the time, and 
the diameter of the true disk in miniature is obtained.* 
The sun's diameter varies, however, somewhat : 
In January it is 32^' sine = -00945 = ^ig. 
„ April „ 32' sine = -00931 = 1 
„ July „ 31i' sine = -00916 = • 
Hence the diameter of a miniature of the sun formed by a 1-inch 
lens will be respectively, in those months : 
T^6' T^85 T^9 iiich nearly. 
And this, reduced by a 3-inch lens instead of a 1-inch, would be : 
TU6» ih, and 
* The diameter of a white wafer on a black-board placed at such a distance 
as to subtend the same angle as the sun at the time would be 1 inch placed 
a 1 106 inches distance. 
