OF OPTICS. 247 



In this hole, place the end of a tube, containing two con. LECT. 

 vex glasses and an object, viz. 1. A convex glass a a, of ^^11^ 

 about two inches diameter, and three inches focal dis- 

 tance, is lo be placed in that end of the tube which is put 

 into the hole. 2. The object b b, being put between 

 two glasses (which must be concave to hold it at liberty) 

 is placed about two inches and a half from the glass 

 a a. 3. A little more than a quarter of an inch from 

 the object is placed the small convex glass c c, whose 

 focal distance is a quarter of an inch. 



The tube may be so placed, when the sun is low, that 

 his rays A A may enter directly into it : but when he 

 is high, his rays B B must be reflected into the tube by 

 the plane mirror or looking-glass C C. 



Things being thus prepared, the rays that enter the 

 tube will be conveyed by the glass a a towards the object 

 b b, by which means it will be strongly illuminated ; 

 and the rays d which flow from it, through the magnifying 

 glass c c, will make a large inverted picture of the object 

 at D D, which, being received on a white paper, will 

 represent the object magnified in length, in proportion 

 of the distance of the picture from the glass c c, to the 

 distance of the object from the same glass. Thus, sup- 

 pose the distance of the object from the glass to be 

 ^ parts of an inch, and the distance of the distinct picture 

 to be 12 feet or 144 inches, in which there are 1440 

 tenths of an inch ; and this number divided by ^, gives 

 480 ; which is the number of times the picture is 

 longer or broader than the object; and the length multi- 

 plied by the breadth, shews how much the whole sur- 

 face is magnified.** 



Note 66. The simplest microscope which can be employed to any 

 nseful purpose is perhaps that which is made with a drop of water, sus- 

 pended in a very small hole in a thin slip of brass, or any similar mate" 

 rial. This may easily be constructed where no other microscope can be 

 obtained, and its performance will be found very satisfactory. A sphe- 

 rule of water, it must be observed, of the same size as one of glass, will 

 not magnify so much as the latter, because, as its density is not so great, 



