270 The Bight Eon. Lord Bayleigh [Feb. 6, 



there must be a limit to definition. The images of the components of 

 a close double point will overlap ; and if the distance between 

 the centres do not exceed the diameter of the representative patches 

 of light, there can be no distinct resolution. Now their diameter 

 varies inversely as the aperture ; and thus the resolving power is 



directly as the aperture. ..i. 4. ^i •„ 



My object to-night is to show you by actual examples that this 

 is so I have prepared a series of photographs of a grating consisting 

 of parallel copper wires separated by intervals equal to their own 

 diameter, and such that the distance from centre to centre is yo-i^cH. 

 The grating was backed by a paraffin lamp and large coudensmg lens ; 

 and the photographs were taken in the usual way, except that the lens 

 employed was a telescopic object glass, and was stopped by a screen 

 pei^orated with a narrow adjustable slit, parallel to the wires. in 

 each case the exposure was inversely as the aperture employed. IJie 

 first [thrown upon the screen], is a picture done by an aperture ot 

 eight hundredths of an inch, and the definition is tolerably good 

 The next, with six hundredths, is rather worse. In the third case, i 

 think that everyone can see that the definition is deteriorating ; that 

 was done by an aperture of four hundredths of an inch, ihe next is 

 one done by an aperture of three hundreths of an inch, and you can 

 see that the lines are getting washed out. In focussmg the plate for 

 this photograph, I saw that the lines had entirely disappeared, and i 

 was surprised, on developing the plate, to find them still visible. That 

 was in virtue of the shorter wave-length of the light operative m 

 photography as compared with vision. In the last example, the 

 aperture was only two-and-a-half hundredths of an mch, and the effect 

 of the contraction has been to wash away the image altogether, 

 although, so far as ordinary optical imperfections are concerned, the 

 lens was acting more favourably with the smaller aperture than with 



the larger ones. , . , , ,^ . 



Thfs experiment may be easily made with very simple apparatus 

 and I have arranged that each one of my audience may be able to 

 repeat it by means of the piece of gauze and perforated card which 

 have been distributed. The piece of gauze should be placed against 

 the window so as to be backed by the sky, or in ^ont of a lamp 

 provided with a ground-glass or opal globe. You then look at the 

 gauze through tht pin-holes. Using the smaller hole, and gradually 

 drawinc. back froni the gauze, you will find that you lose defi- 

 n t'on ^nd ultimately all sight of the wires. That will happen at a 

 distance of about 4.f feet from the gauze. If, when looking throu^ 

 the smaller hole, you have just lost the wires, you shift the caid so 

 as to bring the larger hole into operation, you will see the wires 



^^""Tirt'ls'^^one side of the question. However perfect your lens 

 may be, you cannot get good definition if the aperture is too much 



* The distance between the grating and the telescope lens was 12 ft. 3 in. 



