350 ON THE PROGRESS OF SCIENCE, 



merits, which like the latitude and longitude of a place, are 

 requisite for determining the position of a celestial object, could 

 not be obtained at the same time except by the aid of two 

 observers : and, moreover, the mural circle was open to objections 

 from the mode in which it was usual to mount it. 



Now, however, both these instruments are combined in one 

 under the name of the Transit or Mendian Circle ; so that 

 complete observations of a body are made simultaneously by one 

 observer, and both instruments receive an amount of strength 

 and freedom from mechanical defects which could not be 

 previously obtained. Even in this instrument an improvement 

 is now made, and one which I have adopted with advantage in 

 the small transit circle of the Sydney Observatory. Formerly it 

 was necessary to raise the transit instrument entirely off its 

 bearings, in order to obtain the error of collimation of the 

 telescope. This was at all times a work of difficulty, and 

 oftentimes of hazard ; the necessity for this is now superseded 

 by the simple expedient of having an aperture through the centre 

 of the telescope, which is closed when not in use, and affords an 

 uninterrupted view of one collimator from the other. 



Time would fail to dwell upon the improvements which have 

 been made in telescopes equatorically mounted, and the progress 

 which has been made by opticians and mechanics in the con- 

 struction of lenses, and that delicate apparatus for measur- 

 ing small spaces, known as the micrometer. Professor Airy 

 considers the screw of the micrometer attached to the telescope 

 of the transit circle at Greenwich, the workmanship of 

 Messrs. Troughton and Sims, to be almost perfect. But there 

 is still room for improvement in the graduation of circles, and 

 the construction of lenses. Both indeed have attained to a high 

 degree of excellence : yet the most finely graduated circle is 

 found to be somewhat imperfect, and the optician has not yet 

 succeeded in producing a lens perfectly achromatic. 



Hitherto I have been speaking of instruments especially 

 adapted to the scientific work of an observatory, and in such a 

 case there is probably a limit to improvement, for what is 

 required is stability, clearness of definition, and accuracy in 

 measurement : and these will be sacrificed if we attempt to 



