4 SCIENCE PROGRESS 



it was only after a search of fifteen years that Hencke, an 

 ex-postmaster of Driessen, was at last rewarded by another dis- 

 covery. From that time the number has been extended almost 

 continuously, so that we now know nearly seven hundred of 

 these tiny bodies. From the circumstances attending the dis- 

 covery and the subsequent observation of them has arisen one 

 need for charting the places of the fainter stars. The easiest 

 way to record the movements of these small bodies is to 

 measure their distances from adjacent faint stars, and this is only 

 satisfactory when we know the places of the stars themselves. 

 This led astronomers to undertake the great work of charting 

 the zone of the heavens called the Zodiac, in or near which all 

 the planets move. Such an enterprise was started at Berlin 

 early in the nineteenth century ; another, initiated by Chacornac 

 many years later was continued by the brothers Henry of Paris, 

 who ultimately took the great step of employing photograph}' in 

 the work ; and this led to the inception of the scheme we are 

 now considering. 



The introduction of the photographic method was at first 

 fitful and tentative. Apparently the earliest attempts were 

 made in America by the Bonds and by Rutherfurd. It is 

 curious now to read of the difficulties in obtaining impressions 

 of any but the brighter stars in the old days of wet plates. 

 The wet plate of course was not nearly so sensitive as the 

 dry plate ; also it could only be exposed for a limited time 

 before it dried up, and during such limited exposures only the 

 brightest stars left an image upon it. Even the wildest hopes 

 of these early pioneers in forecasting the future fell far short 

 of what is now easily attainable : witness the following extract ^ 

 from a letter of George Bond to the Hon. William Mitchell, 

 Nantucket, dated from Cambridge (Mass.) July 6, 1857: 



" As far as I am informed, the attempt to photograph the 

 fixed stars by their own light has been made nowhere else up to 

 the present date. The rumor of a daguerreotype of a nebula 

 made in Italy some 3'ears since, was unfounded. . . . 



"About seven years since (July 17, 1850) Mr. Whipple 

 obtained daguerreotype impressions from the image of a Lyroe 



' Memorials of William Cranch Bond, Director of the Harvard College 

 Observatory 1840-59, and of his son George Phillips Bond, Director of the Harvard 

 College Observatory 1859-65, by Edward S. Holden (Lemcke & Buechner, New 

 York, 1897), p. 155- 



