192 WRITINGS OF JOSEPH HENRY. [1841 



ON A SIMPLE FORM OF HELIOSTAT. 

 (Proceedings of the American Philosophical Society, vol. ii, pp. 97, 98.) 



September 17, 1841. 



Professor Henry exhibited to the Society a simple form 

 of the Heliostat, or instrument for throwing a stationary 

 beam of light into a darkened room. 



He stated that this article of apparatus, which is indispens- 

 able in delicate experiments on light, is in its usual form a 

 very complex instrument and consequently very expensive, 

 while the one to which the attention of the Society was 

 directed is very simple, and cost scarcely more than the tenth 

 part of the price of one of the old form. 



It was made in accordance with the plan given by Dr. 

 Thomas Young in the first volume of his Lectures on Natural 

 Philosophy, which consists in reflecting a beam of light into 

 the room in a line parallel to the axis of the earth, and then 

 causing it to retain this direction by giving the reflector a 

 rotary motion equal to the apparent motion of the sun. 

 The instrument consists of a flat block of mahogany, about 

 nine inches long and five inches wide, on which is placed 

 in an inclined position, the wheel-work of a common pocket 

 watch. This serves to give rotary motion to a brass wheel 

 of about five inches diameter, which is so geared into the 

 large wheel of the watch as to make one turn in twenty-four 

 hours. The axis of this wheel is a steel rod, carrying on its 

 upper end a small mirror, which can be set in any position 

 by means of a universal joint. The watch-work and the 

 wheel are attached to the mahogany block by a hinge, so 

 that the axis of the wheel can be inclined to the horizon at 

 an angle precisely equal to the latitude of the place where 

 the instrument is to be used. 



The adjustment of the instrument is very simple. It is 

 placed on the outside of the window, with the axis of the 

 wheel parallel to the axis of the earth; a meridian line 

 having been traced on the window-sill for this purpose. 



