The Photo Visual triplet objective was an optional extra on all the Cooke scopes at that time. The lenses are in excellent condition and I understand this is rare for these objectives.
There is an excellent article on the Cooke Photo/Visual triplet here: http://www.europa.co...pe/hdtaylor.txt
As the lens was known to develop a milky haze, according to this article Cooke triplets needed repolishing about every 20 years. As a result, because the internal element had such steep curves, and became so thin, only 2 or 3 re-polishes were possible before it became to fragile.
My P/V lens (actually a 4"), and it too is clean. I have a theory about this...
The larger Cooke triplets were mounted in a very complicated cell, which involved metal strips that expanded in response to temperature changes, at exactly the same rate as the cell itself. As the cell itself expanded leaving the lens elements loose, these strips would also expand within, and take up the space, thus keeping pressure on the lens, and keeping the objective centred. Genius.
However, the smaller Cooke P/V's were mounted in a modified (but standard) bronze cell...which has two blocks, and a large sprung plate that keeps the lens in place. So my casual theory/thoughts are that perhaps an element in this ultra complex cell was the catalyst for the hazy film that this lens became known for?
Picture of the internals of my 4" P/V below.
Thomas Cooke operated on his own from the 1830's in York, and as a result, early examples of his telescopes bear the name Thomas Cooke/York (as does the one that is the original subject of this thread). Two of his sons joined him in the early 1860's, at which point the company became T. Cooke & Sons. Over time, they opened & closed several retail shops in London & York, and the engravings on the scopes change to reflect the time when these shops were open. They provide good clues to help date Cooke's works.
