316 OPTICS. 



many, or the uniting of many into one, their mag- 

 nitude, decrease, and disappearance. 



An eclipse of the sun may, by the solar tele- 

 scope, be viewed to the greatest advantage, we 

 having it in our power to represent the face of the 

 sun as large as we please, consequently, the eclipse 

 proportionally conspicuous j the circle of the sun's 

 disk may be so divided by lines and circles drawn 

 thereon, that the quantity of the eclipse intimated 

 in digits, may this way be exactly determined; 

 also the exact time of the beginning, middle, and 

 end of it, for determining the longitude. The 

 transits of Mercury and Venus over the face of the 

 sun are exhibited most satisfactorily ; the planets 

 appear truly round, well defined, and very black : 

 their comparative diameters to that of the sun, the 

 direction of their motion, and the times of ingress 

 and egress, are here viewed to the greatest per- 

 fection. 



The heliostata, to take off the inconveniences 

 which arise from the motion of the earth, in mak- 

 ing experiments on the solar light, was an excel- ' 

 lent invention of Dr. Gravesande : it consists of 

 two principal parts, each of which consists of 

 many smaller parts. The first is a plain metallic 

 speculum, supported by a stand ; the other is a 

 clock, which directs the speculum according to the 

 earth's motion, keeping the sun in the same point 

 of view. 



The Magic-lanthorn, and Phantasmagoria. 



The magic-lanthorn is a small machine which has 

 been generally used for amusing children, by mag- 



