i8 PERSPECTIVE GLASSES 



300 feet, and that hjs other telescopes were useless, because on 

 account of .their length, the centres of the lenses cannot be brought 

 into a straight line. He asserted also that lenses of over great size 

 were of no use, and that neither Newton or Marshall here in England 

 could produce anything of value in this kind.' l 



In 1690 John Marshall was practising as a manufacturing optician in 

 the Strand with such success that in January 18, 1693-4, his methods 

 and results received the official approval of the Royal Society. The 

 original letter that he received, signed by Edm. Halley, has been 

 facsimiled in the illustrated edition of Green's History of England, 



P-I575- 



One certainly, and perhaps two, of the oldest know r n specimens of 



his art are in the Orrery Collection at Christ Church. Their many 

 draw-tubes covered with vellum are surprisingly light and smooth 

 in use. 



For an early illustration of a telescope with a paper or cardboard 

 tube cf. Zahn, Ocitlus artificialis, 1685, p. 1 88. The Orrery instruments 

 by Marshall and Wilson appear to be constructed in accordance with 

 the principles set out at length by Zahn. 



TELESCOPES 



89. |-inch Refractor. c. 1700. 



Vellum tube, closing to 25^ inches. Orrery Coll. (possibly 28). 



Marked IOHN MARSHALL, LVDGATE STREET, LONDON. 



The outer body (diam. 3^ inch.) is covered with vellum, blotched 

 green and red and ornamented with book-binding designs stamped in 

 gold. The draw-tubes, of which there are 9, are of cardboard covered 

 with white vellum, and are marked with pencilled rings to indicate full 

 extension. When fully extended it measures 9 feet. The objective is 

 1 1- inches in diam. with an effective aperture of | inch and a focal length 

 of 8 feet. The eyepiece consists of a biconvex lens of 3-inch focus, 

 which is mounted far within the eyecap. Both object and eye lenses 

 are mounted in turned lignum vitae mounts, provided with wooden 

 screw caps to protect the glasses from dust and injury when not in use. 



90. 2-inch Refractor. c. 1700. 



Vellum tube, closing to 30 inches. Orrery Coll. 14. 



The outer body is covered with white vellum ornamented with book- 

 binders' stamps tooled in gojd. The 9 draw-tubes extend to 12 feet, 

 but the instrument has no object-glass. Judging from the diameter of 



1 Uffenbach, October 6, 1710. 



