THE GREAT STAR MAP 5 



formed in the focus of the great equatorial and subsequently 

 from Castor, thus establishing a simple but not uninteresting 

 fact — the possibility of such an achievement. On these occasions 

 a long exposure of one or two minutes was required before the 

 plate was acted upon by the light. . . . 



" Messrs. Whipple and Black recommenced their trials on 

 other images (taken by the collodion process) in March of the 

 present year and they are still in progress. . . . Could another 

 step in advance be taken equal to that gained since 1850, the 

 consequences could not fail of being of incalculable importance 

 in astronomy. The same object a Lyroe, which in 1850 required 

 100' to impart its image to the plate, and even then imperfectly, 

 is now photographed instantaneously with a symmetrical disc fit 

 for exact micrometer measurement. We then were confined to 

 a dozen or two of the brightest stars whereas now we take all 

 that are visible to the naked eye. Even from week to week 

 we can distinguish decided progress. ... At present the chief 

 object of attention must be to improve the sensitiveness of the 

 plates, to which I am assured by high authorities in chemistry 

 there is scarcely any limit to be put in point of theory. Suppose 

 we are able finally to obtain pictures of seventh magnitude stars. 

 It is reasonable to suppose that on some lofty mountain and in 

 a purer atmosphere we might, with the same telescope, include 

 the eighth magnitude. To increase the size of the telescope 

 threefold in aperture is a practicable thing if money can be 

 found. This would increase the brightness of the stellar images, 

 say eightfold, and we should be able then to photograph all the 

 stars to the tenth and eleventh magnitude inclusive. There is 

 nothing then so extravagant in predicting a future application of 

 photography to stellar astronomy on a most magnificent scale. 



" P.S. — I find I have forgotten to allude to two important 

 features in stellar photography — one is that the intensity and 

 size of the images taken in connection with the length of time 

 during which the plate has been exposed measures the relative 

 magnitudes of the stars. The other point is that the measure- 

 ments of distances and angles of position of the double stars 

 from the plates, we have ascertained by many trials on our 

 earliest impressions, to be as exact as the best micrometric 

 work." 



The letter is a remarkable one for the date. The three 

 forecasts of improvement — increased sensitiveness in plates, 

 larger instruments, and better climate — have all been realised 

 within fifty years. There are two mountain observatories in 

 California ; there is a 40-inch lens, nearly three times the size 

 of the 15-inch Harvard equatorial, at the Yerkes Observatory 



