Interference- Bands. 81 



unfavourable to brightness, and therefore to be avoided in 

 practice. The better to understand this, let us suppose that 

 the slit is backed by the sun, and is so narrow that, in spite 

 of the sun's angular magnitude, the luminous vibration is 

 sensibly the same at all parts of the width. For this purpose 

 the width must not exceed ^ millim.* By hypothesis, the 

 appearance presented to an eye close to the slit and looking 

 backwards towards the sun will be the same as if the source 

 of light were reduced to a point coincident with the sun's 

 centre. The meaning of this is that, on account of the 

 narrowness of the aperture, a point would appear dilated by 

 diffraction until its apparent diameter became a large mul- 

 tiple of that of the sun. Now it is evident that in such a case 

 the brightness may be enhanced by increasing the sun's appa- 

 rent diameter, as can always be done by optical appliances. 

 Or, winch would probably be more convenient in practice, we 

 may obtain an equivalent result by so designing the experi- 

 ment that the slit does not require to be narrowed to the 

 point at which the sun's image begins to be sensibly dilated 

 by diffraction. The available brightness is then at its limit, 

 and would be no greater, even were the solar diameter in- 

 creased. The practical rule is that, when brightness is an 

 object, slits backed by the sun should not be narrowed to 

 much less than half a millimetre. 



Lloyd's Bands. 



Lloyd's experiment deserves to be more generally known, 

 as it may be performed with great facility and without special 

 apparatus. Sunlight is admitted horizontally into a darkened 

 room through a slit situated in the window-shutter, and at 

 a distance of 15 or 20 feet is received at nearly grazing 

 incidence upon a vertical slab of plate glass. The length of 

 the slab in the direction of the light should not be less than 2 

 or 3 inches, and for some special observations may advan- 

 tageously be much increased. The bands are observed on a 

 plane through the hinder vertical edge of the slab by means 

 of a hand magnifying-glass of from 1 to 2 inch focus. The 

 obliquity of the reflector is of course to be adjusted according 

 to the fineness of the bands required. 



From the manner of their formation it might appear that 

 under no circumstances could more than half the system be 

 visible. But, according to Airy's principle |? the bands may 

 be displaced if examined through a prism. In practice all 



* Verdet's Legons d' Optique physique, t. i. p. 106. 

 t See below. 



