84 PROGRESS OF SCIENCE. 



are broken and refracted in the direction we choose ; so that 

 we shall see a close or a remote object under whatever angle 

 we please ; and thus at an incredible distance we should read 

 the most minute letters, we could count the grains of sand 

 and of dust, on account of the great width of the angle under 

 which we should see them, for the distance by itself has no 

 direct importance, but the size of the angle has. And so, a 

 child would look like a giant, a man appear a mountain. 

 And likewise for distance ; so that a small army would seem 

 very large, and placed very far would appear very near, and 

 reciprocally. In this manner we could make the sun, the 

 moon, and the stars descend to us so to speak by bringing 

 their figure close to the earth." 



From such a description to the telescope the distance is 

 not very great, and it is quite possible, as some writers have 

 thought,* that he constructed a telescope for his own use. 

 For we must remember that the Egyptians, and the Greeks 

 after them, made some of their astronomical observations by 

 the means of a long narrow tube, called dioptra, which 

 permitted them to isolate a star, and so prevent in a certain 

 degree the reflection and refraction that made accuracy of 

 observation well-nigh impossible for the naked eye. This 

 tube passed from the Greeks on to the Arabs, and Roger 

 Bacon must have known and used it. He does not describe 

 it particularly, but neither does he describe particularly any 

 of the instruments and apparatus in common use which he 

 mentions in Opus Tertium. And as he was on the one hand 

 fully aware of the properties of magnifying glasses, a man of 

 his rare ingenuity might very well have tried to adjust 

 magnifying glasses to the dioptra, and thereby have made 

 some rough kind of telescope. He would soon probably be 

 led to improve it and make it a more serviceable instrument 

 by the insertion into it of a smaller and narrower dioptra, 

 which would slide up and down so as to give a right focus. 

 His recommendation of the reform of the Calendar would 

 not, taken by itself, warrant the belief that he invented the 

 telescope, since ordinary visual observation and mathe- 



* S. Jebb, Wood, Cuvier. Cuvier thought he could even recog- 

 nise in one of Roger Bacon's descriptions, that of the plain microscope. 



