optics. 315 



with a screw pillar to raise or depress its surface 

 for adjusting it properly, according to the focal 

 distance of the lens, or distances of the objects, 

 will be necessary. Its surface should not be flat, 

 but curved to the segment of a sphere, according 

 to the focus of the lens and distance of the objects. 

 The representation of distant objects in this man- 

 ner will afford the highest pleasure and entertain- 

 ment : and if objects in motion, such as carriages, 

 horses, ships, &c, favourably illuminated by the 

 sun, present themselves, their pictures will be 

 formed in the most exquisite manner. 



The solar telescope is a curious instrument, and 

 is applied to use in the following manner : A 

 scioptric ball and socket being fastened against a 

 hole in the window-shutter in a darkened chamber, 

 place the end of a common refracting telescope 

 with its object-plass and eye-glass into the cylin- 

 drical hole of the scioptric ball, and draw out the 

 tube to its proper length ; this being done, the 

 telescope and ball are moved about until it receives 

 the sun-beams perpendicular on the object-glass 

 through the cylindrical hole of the ball ; the tube 

 with the eye-glass is then to be adjusted by moving 

 it either in or out, until the image of the sun, 

 formed on a white plane or screen, is very distinct, 

 large, and luminous. 



In this manner, the sun's face is viewed without 

 offence to the eyes, and whatever changes happen 

 thereon may be most accurately observed ; the 

 spots, which are seldom seen, even when viewed 

 through small telescopes in the common way, are 

 here conspicuous, and easy to be observed, with 

 all the different circumstances of the spots begin- 

 ning to appear, their increase, division of one into 



