Letter from Nevil Maskelyne to Peter Dolland at St Paul's Churchyard

Greenwich Tuesday Night
Oct. 1. 1771.

Sir,

Since I wrote to you to day,
confirming your experiment, I have
found the reason of it. The indistinct-
-ness produced by the red glass placed
within the focus of the eye glass does
not proceed from the colour of the
glass but from the refractions of
the rays as entring & emerging from
the glass combined with its thickness,
which lengthens the focus of the
object glass. A plane transparent
glass of equal thickness would
produce the same effect.


Page 2

[contains a drawing in the top right corner]

Let A1 be the course
of an extreme ray of the
middle pencil proceeding
towards the focus 1
but intercepted at b
by the place glass PG,
the ray will be refracted to
bc and thence to c2, c2 being parallel
to A1 or B1; therefore the point 2 which
is new the point where the image will
be formed is farther from the [?] object
glass than the point 1 is by the space
1 2 and therefore the telescope must
be lengthened in order to see the sun

distinct. I take it that the magnifying
power will be also altered which
might be easily computed, if it was
of any consequence. I tried
the experiment with the greatest magni-
-fying power, and therefore the


Page 3

Space 1 2 being a greater proportion to
the focus of the eye glass the indistinc-
-tness was thereby rendered greater
than it would have been with
one of the lesser magnifying
powers. The space 1 2 by which the
telescope is to be lengthened is = 1/3d of the
thickness of the plane glass, or [paper torn, text missing]
accurately 10/31st of it.

The effect of the place glass is [paper torn, text missing]
same wherever it is placed be[paper torn, text missing]
the object glass & eye glass. [paper torn, text missing]
a convex eye glass was made use of
the effect would be the same, to
lengthen the focal distance. If the
place glass was placed within the
crosswires of a telescope the object & wires could
not be both seen distinct without altering
the position of the wires. I am

Your humble servt

N. Maskelyne


Page 4

To Mr Peter Dollond
Optician
In St Paul’s Churchyard