10!^ THE HISTORY OF THE TELESCOPE. 



iioL be fultilled; tlierefore Gauss, who was particularly fond of doing 

 wliat all the rest of the world believed imx^ossible, straightway did it. 

 There has been only one eltbrt to carry out this suggestion of Gauss, 

 and that forty years later by Steinheil, but it proved a disappointment. 

 A much larger objective made by (31ark a few years ago, of the general 

 form of Gauss's objective, probably does not meet the Gaussian condi- 

 tion, — at least this condition is extremely critical, and I believe it is not 

 asserted tliat the objective was ever thoroughly investigated. It has 

 been the father of no others. 



It is hardly surprising, since none of these forms have any real merit, 

 that the practical optician has, following the line of least resistance, 

 adopted a form which costs him less labor than those heretofore men- 

 tioned and is quite as good. V>y making tlie curve equi-;'onvex the 

 trouble and expense of making one pair of tools is saved, although this 

 would hardly appear a satisfactory i-eason for choice of a particuhir 

 form to the astronomer, who simply demands tlie best possible instru- 

 ment of research. 



The reason for so much futile work on the theory of the telescope 

 objective is not far to seek. It had always been tacitly assumed that 

 the condition of color correction, one of those which serves to determine 

 the values of the arbitrary constants, was readily determinable — in 

 fact, one of the doniie of the problem, wliereas it is just this datum 

 which has offered peculiar difficulties. Fraunhofer brought all the re- 

 sources at the command of his genius to bear upou this point, and frankly 

 failed, although in the effort he made a splendid discovery, which has 

 assured a permanence to his fame no less than that of the history of 

 science itself — the discovery of the dark, or Fraunhofer, lines iu solar 

 and stellar spectra. Gauss i)roposed the condition that the best objec- 

 tive is that which produces the most perfect concentration of light about 

 the place of the geometrical image of a point, just as the best rilie 

 practice is that which produces the niaxinuim concentration of hits 

 about the center of tlie target. That this is a false guide appears at once 

 from the consideration that if we take even as much as 10 per cent of 

 the light from an object and diverted from the image so far that it can 

 not be found, tlie telescope may still bo practically perfect; all of Her- 

 schel's did much worse than this. But if you take that same 10 jjer 

 cent and concentrate it very close about the image, the telescope will 

 be absolutely worthless. 



The true difficulty with most of the theorists is this: There is no 

 recognition of the relative weight or importance of unavoidable errors. 

 The optician is <'onfronted at the very outset by the fact that absolute 

 elimination of color error is impossible, for certain physical reasons 

 which we have not time for considering farther. He can reduce the 

 color error of the old single-lens type of telescopes hundreds of times, 

 and hence the length of the telesco]>e tens of times. It is tliis fact 

 which prevents the still farther sliortening of telescoiies, which keeps 



