GORHAM, ON THE 



of an intensity varying with the number of apertures employed 

 to render them visible. 



This liglit, be it observed, is always simply transmitted ; 

 never being reflected nor refracted by the intervention of 

 glass, or of any other substance having catoptric or dioptric 

 effects. The apparatus required in these experiments there- 

 fore is essentially very simple and uncomplicated, consisting 

 indeed of a mere short tube open at one end, and having a 

 plane, perforated with small holes to admit the light, at the 

 other ; while the object to be examined is applied at tlie open 

 end, and held as close to the eye as possible. Hence a pill- 

 box and a narrow slip of glass constitute all that is really 

 necessary to explain the laws which are in operation, and to 

 give an idea of the phenomena which are involved in their 

 successful application. 



From the remarks made in a previous section, it is obvious 

 that these phenomena are owing to the size, and to the 

 intervals of the apertures themselves, and to their distance with 

 respect to the eye. Thus their size should be the one-fortieth 

 of an inch, — their intervals the one-tenth of an inch, — and their 

 distance from the eye from one to two inches. If these con- 

 ditions are not fulfilled, the images become either undefined 

 or dimly illuminated ; but when they are strictly observed, the 

 combinations, which an instrument constructed upon such prin- 

 ciples is capable of presenting to the eye, are very beautiful. 



But a mere tube of pasteboard, however well it might 

 answer for a first experiment, is inefficient, for it is difficult 

 to retain the slip of glass in its proper position between the 

 tube and the eye. It was essential therefore to construct a 



small instrument of some more solid 

 and durable material, such as wood 

 or ivory, with which experiments 

 might be conveniently performed. 



This instrument consists of a tube, 

 T, one or two inches long, and about 

 one inch thick, expanding at the end 

 to which the eye is applied into a 

 circular lip, L, which is about one 

 inch and a half in diameter, while 

 that of the round opening in its centre 

 is about half an inch. This end is 

 provided with a slit sufficiently large 

 to permit a thin narrow slip of glass, 

 about an inch broad, to slide easily 

 through it, as shown in the figure. 

 The other end has a circular rim, R, 



Fis. 14. 



