BY A. B. BIGGS. 23 



circumstances as to the state of the air, etc., is 100 to the inch 

 of aperture. This is the extreme, rarely reached. The Lick 

 Telescope has an object glass 36 inches in diameter. Consider- 

 ing the necessarily immense thickness of glass through which 

 the light has to pass, we may assume 3,000 at the utmost of 

 its power. This should, if definition were perfect, give us a 

 view of the moon as at a distance of 80 miles. Of course this 

 is a vast approach. But what could we see of the details of a 

 terrestrial mountain, supposing we could discern it at 80 

 miles distance ? This estimate is, of course, on the supposi- 

 tion that we could eliminate all optical imperfection and 

 atmospheric disturbance, both of which are impracticable. In 

 connection with this question I made the following experi- 

 ment the other day : — I approached a handbill, of the subject 

 of which I was ignorant, until I could just make out the 

 boldest type. I found the distance 16 yards. The letters 

 were bold block letters, 1 inch in height. Reckoning from 

 this, similar letters on the moon's face, to be seen by natural 

 unassisted vision, should measure 417 miles. There would be 

 room across her face for a word of five letters. With a tele- 

 scopic power of 3,000, and no imperfection introduced, such 

 letters, to be readable, would have to be 244 yards. Each 

 letter would thus cover a space of about 12 acres. Allowing 

 for imperfection, I think this would be the smallest patch 

 that would be discernible at all as to shape. At the distance 

 of Jupiter this object would need to be about four million 

 times larger in surface, or, say, three times the size of 

 Tasmania. 



Photography. 



There is one direction, however, in which the powers of 

 the telescope are in process of development to a considerable 

 extent, that is, by its association with photography. I have 

 so far dealt with the telescope as a seeing instrument. What- 

 ever is to be seen is seen at once. We cannot increase its 

 vividness or distinctness by prolonged gaze. If a faint star, 

 e.g., is not at once visible when the eye is directed to its posi- 

 tion, any amount of staring at it will not bring it into view. 

 Not so with photo grapliy. The more it looks the more it sees. 

 In this way it has been arranged to explore the entire 

 heavens to depths far beyond the limits of telescopic vision 

 by combined effort of all the principal Observatories in the 

 world. Already vast numbers of minute stars have impressed 

 themselves upon the photographic plate that would probably 

 for ever have remained beyond the power of actual vision. 

 To accomplish this something more is necessary than merely 

 focussing the object upon the photo plate. It has to be kept 

 there accurately during the whole time of the exposure — a 

 needle's point to a spider line— and that for hours, against 



