Mystery Zeiss Lamphouse 250 Collector

Hello eveyone, 

I have a question concerning the collector lenses of the Zeiss 
Oberkochen Multipurpose Lamphouse 250 (or "pumpkin" lamphouses), which 
despite my initial research has so far has remained a bit of a mystery.

I have three different lamphouse 250s, all with the older grey 
hammertone paint, and all of which are identical except for the 
collector lens found within one of the lamphouses.

Two of them have the what I believe to be the standard collector lens 
found in most of the Lamphouse 250s (also seen in this line drawing):

http://www.sunley.ca/microscope/images/collectorlensdrawing.jpg


.. but the third has a different collector lens.  Technically it seems 
to actually have the same glass lens elements as the others, but the 
metal cylinder which houses the lenses is quite different.  The cylinder 
is much shorter in length, and unlike the one pictured in that drawing 
which positions the lenses very close to the bulb, this shorter cylinder 
cannot possibly bring the lenses that close to the bulb.  

I have pictures of the two types of collector lens sets side by side:

http://www.sunley.ca/microscope/images/collectorlens1.jpg
http://www.sunley.ca/microscope/images/collectorlens2.jpg
http://www.sunley.ca/microscope/images/collectorlens3.jpg


When installed in the lamphouse, the shorter cylinder puts the 
lenses approximately 20 mm further away from the bulb then the 
longer cylinder does.

Although it's hard to tell from the pictures, both cylinders are made 
from brass, and have the same brownish-black colour.  They also use an
identical retaining ring and spacer to keep the lenses positioned. 
Although another difference is the lenses in the shorter cylinder seem 
to be optically coated, while the longer cylinder's lenses are not.

Has anyone ever come across this type of shorter collector lens?  So far 
I have determined that atleast two collector lenses were available for 
the Lamphouse 250, a conventional glass lens, and a quartz collector 
lens. Also apparently there were adaptions to make the lamphouse 250 
compatible with the Axiomat microscope (but I've heard this did not 
require a different collector lens), as well I've found there are third 
party companies which are/were producing lenses for Zeiss lamphouses 
(although I do not know if they produced any for the Lamphouse 250).  

Both sets of lenses seem to be of the same optical design (similar 
sized and shaped lens elements, although I haven't compared them 
beyond that), and also have the same light transmission cut-off of 
around 300nm (therefore this does not seem to be one of the quartz 
collectors).


While I initially thought that it's possible the lenses may have been 
for different types of bulbs (arc lamps vs. filament bulbs, etc), but it 
seems strange that the Zeiss drawings would not indicate that different 
lenses were available.  As if it were for different bulbs, I suspect it 
would have been recommended in their literature to use the proper lens 
set for each bulb type.  Also, as these seem to be the same lenses 
optically, just spaced further away from the bulb, it would just focus 
the image of the bulb at a much closer distance (as the closer the 
collector lens is to the bulb, the closer to infinity the image of the 
filament would be). 

I've found that moving the conventional (longer) lens of the lamphouse 
250 to a distance of where the shorter lens would place the lenses 
results in focusing the bulb approximately 15 cm from the front of the 
mounting bracket of the lamphouse 250.

Does anyone have any ideas as to the purpose of this shorter collector 
lens, or of any Zeiss equipment of that vintage which would require the 
bulb to have been focused so closely?

Thanks in advance,

Kevin Sunley

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