Astronomical and Nautical Collections, 135 



angle of about 32'; that is, the rays coming from the opposite 

 extremities of any of its diameters make with each other an 

 angle of 32'. Now, in order to determine the magnitude of 

 the image, we have only to follow these rays through the 

 centre of the lens, till they arrive at the distance of the 

 focus, that is, -f^J of an inch, and the chord of a little arc 

 of 32' in extent, of this radius, will be about yu^^h of an 

 inch, that is, sj^: and such will be the diameter of the 

 little image of the sun formed in the focus of the lens, by 

 the rays thrown by the mirror upon its surface, which, 

 after crossing in this point, will diverge into a luminous cone 

 much more extensive than that which is formed by the sun's 

 rays simply admitted into the hole, especially if the lens is 

 of tolerable magnitude. This extension of the cone gives a 

 great facility to the experiment ; it was first pointed out to 

 me by Mr. Arago, and I have since always used it in my 

 experiments. It is also necessary to take the jDrecaution of 

 excluding the direct rays of the sun from the surface of the 

 lens, in order to avoid the confusion that might arise from 

 having a second luminous point producing a distinct series 

 of phenomena. 



If it is desired to have the luminous point absolutely fixed, 

 as in the case of the accurate measurement of the relative 

 situation of the fringes, it will be proper to employ, instead 

 of a simple mirror, a heliostate, an instrument so called, 

 from keeping the reflected ray in a constant direction, not- 

 withstanding the diurnal motion of the sun. It is easy to 

 perceive, that without this precaution the reflected rays, 

 changing their direction with that of the incident rays, 

 would cause a small change of place in the image which is 

 formed by their convergence. But this perfect immobility 

 of the luminous point is only necessary in the case of our 

 wishing to measure the exact dimensions of the fringes : and 

 even for this purpose we might dispense with it, by taking 

 only a few measures at once ; so that each operation might 

 be performed in a short time, and by employing a lens of a 

 short focus only. 



Having described the best manner of forming a luminous 

 point, I shall explain the most convenient mode of measuring 



