10 Mr. Barlow on the Refracting Telescope. 



it is still practically existing in the telescope ; that is, we can 

 only destroy the aberration for one index, whereas the perfec- 

 tion of the instrument requires it to be destroyed for three ; 

 an error, therefore, must remain, and it is for this reason that, 

 with a given aperture, opticians are compelled to employ a 

 certain length of tube, or rather, not less than a certain length ; 

 and, unfortunately, that length increases in a higher ratio than 

 the aperture. The only remedy, therefore, is a reduction of 

 the curvatures; and in some cases included in the above 

 classes, these curvatures may be reduced to one-eighth of 

 those of the common telescope of the same focal power ; but 

 to what extent we may in consequence reduce the length 

 of the telescope, can only be satisfactorily determined by 

 experiment. 



Unfortunately, such experiments are attended with great 

 expense ; and that expense is rendered greater than is neces- 

 sary, because many of our observers, however competent they 

 may be to judge of the performance of a telescope, have too 

 little inclination to examine theoretically and judge of prin- 

 ciples ; and therefore to gain their assent to any new form of 

 construction, the instrument must be perfect, which of course 

 requires the most perfect glass and the best workmanship, and 

 necessarily creates great expense : whereas, if they were able 

 or content first to examine principles only, the charge of such 

 experiments would be much diminished ; and when the best 

 principle had been selected, then, and not till then, I would 

 incur the expense of perfection. 



It is exceedingly difficult, with such a field of inquiry before 

 one, to say which of all the different cases should be selected 

 as an individual test. I have, however, suggested a form, and 

 the Royal Society have ordered the instrument to be con- 

 structed, which I have every reason to hope will be found 

 highly advantageous. In this, the amount of spherical error, 

 or, which I consider equivalent, the amount of curvature, 

 will be less than one-eighth of what would belong to the com- 

 mon telescope of the same focus. The focal power will exceed 

 the focal length, and the sum of all the refractions in the 

 passage of the rays through the lenses, as well as at each in- 

 dividually, will be reduced to a minimum.. These appear to 



