NavList:

One book, David, does mention it, on page 105 of "The Nautical Sextant", written, modestly, by me. For those who have not yet been wise enough to buy a copy, plain glass of refractive index of 1.55 at an angle of incidence of 10 to 15 degrees, typical of most sextants, reflects nearly 9 percent of the incident light.  Bill Morris

Bill

Well, after that sales pitch, I had to buy the book.  Kindle edition only, my bookshelves are full.  The only problem with Kindle is finding a page number.  How much you see seems to vary with the size of your PC/smartphone.  Please give more clues, e.g. Chapter, or between which diagrams.

Actually, while searching to find which of your publications I’ve already downloaded and now lost inside my PC, I came across this on page 8 of ‘My Modern Sextant’ by Heath & Co,  price one shilling and sixpence.

“While on the subject of Horizon Mirrors it may be pointed out that the reason there is an unsilvered portion is to enable exact contact to be made between the two objects observed; because the unsilvered portion partially reflects the reflected object.”  DaveP