130 



PROCEEDINGS OP THE AMERICAN ACADEMY 



of which was not destroyed by the grinding, we have a first surface 

 reflector which will bear a high magnifying power if the mirror be not 

 strained in mounting. 



The scale is read in a novel manner. The cell in which the mirror 

 swings is closed by a bit cut from the centre of an ordinary spectacle 

 lens of from 36 to 48 inches' focus. The scale is located as in tlie 

 ordinary arrangement of telescope and scale, but instead of the tele- 

 scop^we have the following device. A tin or paper tube is provided, 

 and in it, at a distance of five inches from one end, is placed a lens, 

 three inches in diameter, and of six inches' focus. The end of the 

 large tube has a draw-tube, which is closed by a metal plate provided 

 with a peep-hole at its ceutre. The peep-hole may be made to occupy 

 the focus of the lens by sliding the draw-tube backward or forward as 

 required. A horse-hair or very fine wire is stretched across the tube 

 in the focus of the lens opposite the draw-tube, so that it may be 

 clearly seen by the eye applied to the peep-hole. This arrangement 

 is to be mounted in the place occupied by the ordinary telescope, 

 which it far surpasses both in definition and in the breadth of the field 

 of view. The whole apparatus when in position opposite to the gal- 

 vanometer constitutes a telescope, of which the lens in the galvanome- 

 ter is the object-glass, the large lens in the tube the eye-piece, and 

 the scale the object viewed. The definition is quite remarkable, the 

 fibre of the paper upon which the scale is ruled showing clearly, as 

 though seen through a magnifying-glass. 



The details of construction of the entire apparatus having thus been 

 outlined, it remains to speak of the action of the whole. As an ex- 

 ample of the accuracy that may be expected in the performance of the 

 instrument, the following test may be taken. 



October 23d, 1888, the sun was taken as a source of heat, the rays 

 being reduced by passage through a small opening. The following 

 galvanometer deflections were obtained : — 



Divisions. 



First 162 



Second .... 162 



Third 161 



Fourth .... 162 

 Fifth 162 



Divisions. 



Sixth 164 



Seventh .... 162 



Eighth 161 



Ninth 161 



Tenth 162 



The observations were taken rapidly, occupying less than ten min- 

 utes in all. 



No great effort has been made to secure superior sensitiveness, yet 

 the heat from the face of a person at a distance of fifty feet is measur- 



