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Don,

> I am a bit confused.  I thought that a simple two lens telescope inverted
> the image, and a three lens telescope produced an erect image.

There are two types of two-lens telescope:

a) Galilean (straight) and
b) Kepler (inverting).

The difference is in the smaller lens (eyepiece). It is positive in Kepler
and negative in Galileo.

> In nautical literature of the 18th and 19th century,  I have come across
> references to naval officers using a 'night glass'.

"Night glass" refers to a telescope with large objective lens.
It gathers more light than a naked eye.
Most of them are Galilean, because of the reason I mentioned:
with the same diameter of the objective lens, the Galilean is shorter.

Of course, it can be used day and night.

Alex.

> The ordinary day time
> telescope was a terrestrial one, with an erect image.  After sunset, they
> might use a night glass with one less lens, which gave an inverted image
> but more light transmission.
>
> Don Seltzer
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