THE IIIbTOKY OF THE TELEiSCOPE. 100 



the ratio of length to (liaiiu'tcr not less than tiftecn to one in large tele 

 scopes. This restriction being recognized, let us re\ ise our limiting coir 

 ditions. They now become, first, lixed fo(;al length; second, l»est color 

 <'orrection; third, freedom from spherical aberration for a paiticular 

 wave-length of light. We therefore have still one arbitrary constant 

 undetermined. How shall we tix its value, and thus solve tiie problem 

 comi^letely ? Surely there is only one rational guide. Consider the 

 residual errors and make the fourth condition such as to reduce these 

 errors as far as possible. Xow the only remaining errors are secondary 

 color error and spherical aberration for colors other than that for which 

 it is eliminated, or more scientitically, chromatic difference of spherical 

 aberration. Which of these is the gravest defects Our answer must 

 de|)end upon the use to which the ol)jective is to be put. If it is a 

 high-power microsco[)e objective, it is certainly the second. If it is an 

 objective to be used for photographing at considerable angular dis- 

 tances from the axis, our ({uestion loses its ])hysical significance, since 

 we ha\ e excluded the consideration of eccentric refraction. But if the 

 ol)jective is to be for a visual telescope, there is no ([uestion that the 

 defect of secondary color error is incomparably the most serious. Our 

 fourth and determining condition must, therefore, be better color cor- 

 rection. 



These are therefore the consideraf ions whicli have served as guides 

 in the construction of the Carleton College objective. First, the selec- 

 tion of the materials which, in the present condition of the art of opti- 

 cal glass making, possess in the highest degree the desired physical 

 l)roperties; second, a general discussion of every possible combination 

 of these two pieces of glass and a selection of the forms whi(;h yield 

 the best at tainable results. This conscientious strife after scientihc 

 perfection, the unexcelled skill with which the results of analysis have 

 been interpreted into the reality of substance, the gratifying identity 

 of pre<licted and realized values of ]>hysical characteristics — all of these 

 have led some of those v. ho have watched the growth of this new in- 

 sti'ument of research with the most solicitous attention to the belief 

 that although not tin' most powerful in existence it nu»y well be the 

 m(»st jx'rfcct great telescope yet made. Let us thcretbre (congratulate 

 the possessors of this m)ble instrument, wish them (iod s])eed in their 

 search after knowledge, wliile we remiml them that although no as- 

 tronomer can ever n»ake another discovery which will rival that made 

 by the insignificant tube lirst directed toward the heasens by the 

 i'aduan philosophei', yet no mind can weigh the imj)ortance of any 

 truth, liowever trivial in ap])earance, which may be added to that store 

 whicli we call "science."' 



