All casket sets are hard to find and/or expensive. Finding single cells and incomplete sets is easier, but in my experience these never fit the barrel you have - and I have lots.
Hermagis seem to have produced casket sets over a period of more than 30 years, so both optical and mechanical changes are very likely - meaning that the chance of an orphan cell fitting another set is quite unlikely.
There also seems to have been at least two very different optical designs used, which means that the quality of a single cell from one set says very little about the same from another set.
Attached is the table from the Hermagis 1925 Catalogue so hope it helps.
Hollis, I don't think I have a table for your set, but with a berthiot set I found a
cheque stub with the focal lengths, and it showed the maths involved.
The focal lengths are added together Say 390+305=695 this is then divided by 4 which gives 173.75 it seem close enough.
The correct maths is Ft= 1/( 1/F1 + 1/F2 + d/F1*F2), where the F1 and F2 are the focal lengths of the cells, Ft is total focal length, and d is a spacing correction factor. The last term has been written in very many different ways through the ages, depending on how and by whom the spacing d was measured.
Ft=(F1+F2)/4 is wrong, but will give an approximately correct answer for two very similar focal lengths with no spacing.
Thanks for the correction, maths is not a strong point with me, I am sure I have number blindness as well as word blindness.
Thanks and Regards
Sven Schroder, Ole, Thanks!
And by the way: They are 3 elements to each cell, looks like a Dagor-type design at a casual glance.