20 



TELESCOPES 



This type of instrument was figured in the Phil. Trans, for 1667. 



Wilson's microscope lenses were famous, but there cannot be many 

 signed telescope object-glasses now in existence. So far as we know, 

 this one is unique. It is certainly unusual for so long an instrument 

 to be fitted with an erecting eyepiece. 



93. i^-inch Refractor, of about 7 feet 9 inches focal length. 



Orrery Coll. 16. 



Lenses mounted in brass mounts at the ends of the 3-inch square 

 mahogany tube. A sliding cradle with trunnions was used to carry the 

 tube on a tripod stand, but additional steadiness and support was gained 

 by a cord attached to a brass C-hook near the eyepiece end, a support 

 necessitated by the great weight of the heavy micrometer that was used 

 with this instrument. 



A wooden bracket bearing two wooden 3-inch pulley wheels was 

 found among Lord Orrery's apparatus, which may have been employed 

 for supporting his telescopes. 



94. Micrometer Eyepiece for ditto. c. 1700. 



Orrery Coll. 6. 



Probably one of the oldest micrometers now in existence. 



Gascoigne used a thread micrometer as early as 1640, but his method 

 was so little known that even in 1717 Derham considered the facts of 

 Gascoigne's invention worthy of a letter to Crabtree, which was printed 

 in the Philosophical Transactions. This must have been about the 

 date of the Orrery instrument. Gascoigne's micrometer passed into 

 Townley's hands, and he improved it by adding a double threaded 

 screw. 



In 1666 Auzont and Picard also devised a thread micrometer, and f* 

 Romer in 1676 used thirteen silk threads for the same purpose. 



95. Two Square Oak Telescope Tubes. 



Radcliffe Observatory. 



A. 8 feet tube, 3 inches square for 2^ inches obj. Interior octagonal. 



B. 2-draw 7 feet tube, 3^ inches square for 2| inches obj. gl. of 

 13 feet focal length. Also a 5 feet 4 inches extension tube, making 

 a total tube length of 18 feet 6 inches. 



96. Telescope Stand. I7 _. 



Radcliffe Observatory. 

 Mahogany. 3-draw, triangular section, with brass pivot at top. 



97. Mahogany Support for Eyepiece-end of Telescope. 17. 



Radcliffe Observatory. 

 With altitude screw and azimuth rack. 



