French Lens Conundrum - Photographic or Telescope?

[Steven Tribe]

I think french Ebay has rather more interesting items than the UK/USA sites at the moment. I couldn't resist this set of 4 alternative lenses. Two of these were "plus grande" and two were "plus petit". It is exactly the text which appears on the (often missing!) extra lenses which are mounted inside the Derogy " Foyers multiples et lentilles additionelles" - see secon image. These are Bayonet attached lenses too, but I didn't think this was part of a huge, more complicated version of this Derogy.

The lenses were marked 43 and 55 for the "petit" and 86 and 108 for the "grand". I assume the normal lens they replace is something like a "70".

I was expecting a set of single lens, but they turn out to be achromats. The numbers certainly don't represent focal length in cm. Perhaps inches. Combining the two shorter lenses I was able to produce an image at around 100cm.

The Fvalues and focal lengths are so high that I feel these must be replacement/alternative front achromats for a telescope, rather than for photography. Anyone with any insight into sophisticated French telescopes around 1870 - 1890?

[seven]

I was thinking about buying them too, but then sanity kicked in. I don't think i will ever find the Derogy to fit them and even if i do i can not afford it.
These are most certainly from a BIG Derogy Foyers multiples lens. There is a big-ish one right now on ebay (262256264385) and the "lentilles additionelles" look almost exactly like yours. Also the engraving is the same as on other Derogy "lentilles additionelles".

[Steven Tribe]

I would love them to be that but:

1. The focal lengths don't make sense for the central lens. They would include -ve lenses, too.
2. These extra lens in the Derogy set are simple symmetrical lenses (+v and -ve) and not well made achromats like mine.

But I agree about the acute similarity of the engraving with that on the current Derogy lens! Two of the lenses have an identical second engraving of which only about the bottom half remains, which means the cell mount was originally made broader and the cut down on a lather before the final engraving was added. Perhaps not an everyday item.
I'll see if they made a range of large telescopes during the same period.

There may be some clues in the other optical items that were listed at the same time. There was a no.1 Eidoscope which was described as a parts Telescope and the odd lot I bought was linked to a telescope, too.

[seven]

Another possibility would be that they are part of a larger "trousse" set. I've seen such with bayonet mount from Hermagis, but never from Derogy.
What is sure imo is that it is a Derogy product.
About the telescope thing, imo the seller was ignorant, saw some big brass tube with lenses and assumed it has to be a telescope and then assumed that all the rest of the lot is also from telescopes. All of his auctions were with "Telescope ?" in the description.

[seven]

According to D'Agostini (p.344) the "plus petit" of a Derogy Foyers multiples is a positive achromatic doublet. Nothing about the "plus grande".

[Struan Gray]

Good telescopes change the magnification by changing the eyepiece. Changing the main objective would be expensive, and risk upsetting the fine-tuning of the collimation. This was true in the 1880s too (I have a late C19th Zeiss wide-field refractor, and there's no way you'd go messing about with the front objective just to get a slightly different field of view).

[Steven Tribe]

I am sure you are right Struan! I was thinking of microscopes with their turret objectives and fixed oculars.

The only extensive Derogy catalogues for this period seem to be at the well known little exclusive shop in Paris!

The 43 lens has edge markings with o.55. This one also has 180 scratched on the brass after the engraved 43. The value o.55 in dioptres in very close to to 180cm, which is also the focal length I measured for this lens to-day in January sunlight!

These are big lenses - 3 inches of glass - but the fastest of them still only makes F24.

[seven]

the "plus petit" of the lens in D'Agostinis book is 0.6 dioptres.

[Steven Tribe]

So it is!

I am somewhat ashamed to admit that the lenses marked "plus grand" are, indeed -ve achromats. Not easy to identify, except by introducing them in front of a "normal" lens which "hocus pocus" increases its focal length.

A number of things fall into place, I think!

- the figures on each of the 4 lenses is the focal length of the Derogy multiples when that lens is installed in the middle position.
- this was in a casket something like the usual casket, but for 4 lenses instead of 2. The lenses have edge engraving because this is the only way they can be identified at a glance. Side engraving is OK when there are just 2 lenses.
- these extra lenses are for a Derogy Multiples which has a focal length of around 65cm. Which is a truely splendid size.

To my credit, I can claim that I was misled by the optical diagrams in both sections in Corrado's book which show a very simple type of extra lens.

[seven]

i agree, the diagrams in the book show simple lenses.
now you only need to find that BIG Derogy lens