THE MICROSCOPE: CHARLES SINGER 53 



letters far better, and they will seem larger to him. For, according 

 to Canon 5 (see Fig. 1) concerning a spherical medium beneath 

 which the object is placed, the centre being beyond the object, the 

 convexity being towards the eye, all causes agree to increase the 

 size, for the angle in which it is seen is greater, the image is greater, 

 and the position of the image is nearer, because the object is 

 between the eye and the centre. For this reason such an instrument 

 is useful to old persons and to those with weak eyes. For they can 

 see any letter, however small, if magnified enough. But if a larger 

 segment of a sphere be employed, then, according to Canon 6 



Eur IV R&rer t^edtum. 



Fig. 1 



(Fig. 2), the size of the angle is increased, and also the size of the 

 image, but propinquity is lost because the position of the image is 

 beyond the object, the reason being that the centre of the sphere 

 is between the eye and the object seen. Therefore such an instru- 

 ment is not of so much use as the smaller portion of a sphere." 



" Objects are greater when the vision is refracted; for it easily 

 appears by the above-mentioned canons that very large objects may 

 seem to be very small and conversely, and those at a great distance 

 away may seem very near and conversely. For we can so form 

 glasses and so arrange them with regard to our sight and to objects 

 tha^ the rays are refracted and deflected to any place we wish, so 

 that we see the object near at hand or far away beneath whatever 

 angle we desire. And so w^e can read the smallest letters or count 

 grains of sand or dust from an incredible distance, owing to the 

 magnitude of the angle beneath which we see them, while we can 

 scarcely see the largest objects close at hand, owing to the smallness 

 of the angle beneath which we view them ; for distance does not affect 

 this kind of vision save 7?er accidens, but the size of the angle- 



