[4 } 
IV. On Micrometer Telescopes. By Ez, Watkzr, Esq. of 
Lynn, Norfolk. 
To Mr. Tilloch. 
Dear Srr,— qs Dr. Brewster’s ingenious treatise on new phi- 
losophical instruments, there is a note on page 59, respecting 
my micrometer, described in the xxxviiith volume of the Philo- 
sophical Magazine, which is not perfeetly correct. I have, 
therefore, to request the favour of a place in your Magazine, to 
state the subject more explicitly than it seems to have appeared 
to the learned author. 
On page 59, Dr. Brewster gives ‘‘ A Description of an Eye- 
piece Wire Micrometer,” with the following paragraph as a 
note: , 
<< An instrument of nearly the same kind (says Dr. Brewster) 
with that which is the subject of the following chapter, has been 
described by Mr. Ezekiel Walker, in the Philosophical Maga- 
zine for August 1811, vol. xxxviii. p. 127, as a’ new invention 
of his own. I certainly cannot suppose that this ingenious 
writer had an opportunity of seeing any of the instruments of 
this kind which had been constructed under my direction. So 
early as the end of the year 1805, I sent a drawing and a de- 
scription of the eye-piece micrometer to Mr. Cary, optical- 
instrument maker, London. In 1806, one of the instruments 
made for me by Messrs. Miller and Adie, Edinburgh, was ex- 
amined by Professor Playfair; and since that time it has been 
in the possession of a friend in London. The only variation 
in the instrument proposed (constructed) by Mr. Walker, is the 
use of lines on a slip of mother-of-pearl, instead of the silver 
wires.” ' 
Now the Doctor’s micrometer (if I understand it) is founded 
on the following property of the telescope, viz. when the eye- 
tube, containing two glasses, is drawn out, the magnifying power 
of the instrument is increased; and this is the same principle 
on which mine is constructed. This property seems to have 
been discovered by Dr. Brewster himself; but it is not new, for 
an aecount of it was published many years ago. 
In the year 1771, the late Mr. Benjamin Martin published a 
description of a new optical instrument which he calls ‘* Mzcro- 
scopium Polydynamicum: or anew Construction of a Microscope, 
wherein a variety of magnifying powers is communicated to each 
object-lens.”’ 
This microscope with its description I had from Mr. Martin’s 
shop, No. 171, Fleet-street, London. Consequently, I have 
been many years acquainted with this property in optics, which 
affords 
