174 PEOCEEDINGS OF THE AMERICAN ACADEMY 



scope. We used the reflection from the silver surface of the mirror, 

 as affording us more light. 



The advantages of this instrument lie in its simple construction and 

 the ease of its accurate adjustment. The angle of declination in rela- 

 tion to the North star on that part of the earth in which the heliostat 

 is to be used being found, the mirror A is adjusted to throw its central 

 ray R of reflected sunlight upon the centre of the mirror B ; conse- 

 quently, as the mirror B is fixed in the axis of the instrument, the 

 beam of light must necessarily travel along that axis, and the error 

 caused by the motion of the earth is thus compensated as accurately as 

 if the angle of declination were calculated upon the arc of the circle 

 whose radius is equal to a line drawn from the centre of the surface of 

 mirror A to that of mirror B. 



In the course of my experiments, the following observation has been 

 noted : — 



It has generally been supposed that the human eye is not sensitive 

 to that part of the solar spectrum which contains rays of a higher 

 degree of refrangibility than H H. To show that this phenomenon is 

 not constant, the following observation is offered : If the solar spectrum 

 be projected by a converging lens, and received upon tlie retina of the 

 eye at its proper visual focus, the extra-violH rays and their solar lines 

 as far out as L are visibly distinct. 



To make this phenomenon more apparent, let the image of the spec- 

 trum be received upon an opaque paper screen, and from which a hole 

 has been cut out, so that only that portion of the spectrum between H 

 and L may pass through and be received upon the retina of the eye. 

 A little practice in receiving the image at its proper focus will produce 

 the effect above alluded to. 



