C(?7o] 
Mie obje(9:ive glafs will be in lomuch that the apertures are 
a fubduple reafon to the lengths, it folio weth, thataTe- 
lelcope of 3 o foot requiring an aperture of j inches, to an o- 
ther of goo foot, that will bear no more then 9 inches and 
a half, is as i to g . that is, will magnify but 3 times as much, 
but if it fhould make i o times as big, it would rquire a length 
of 3000 foot, which he thinketh not to be practicable. 
4?^. There are ftilklbme irregularities in the nature of 
refra<Stions,as is proved by Mr. Newton^ experiments of colour, 
yet as far as is known, the faid reafon of the apertures to the 
lengths is thereby confirmed. Finally, for the better ufe of 
this Telefcope, he giveth an admonition, that to obferve 
the Satellits of Saturn found out by Cajjinus^ it is convenient 
to look through a narrow aperture, to exclude the light 
that cometh from the fides, and to reftrainthe apple of the 
Eye, which in the dark ufe to be very large and open. 
O X F O RT>, 
Printed at the THEATER, and are to be fold by Mofes T^itt, 
at the :Angel^ and Samuel Smithy at the Princes Arms in St. 
^mfs Chfirch'prd LONDON^ 16^4. 
