If your microscope uses 6.25 base, 50 W, 12 V tungsten-halogen bulbs (my Nikon Optiphot does), and particularly if it supports Koehler illumination but the lamphouse does not incorporate a mirror to effectively double the width of the tungsten filament (my Optiphot does not have such a mirror), then you may want to consider buying the Zeiss bulbs form Ebay (link to the item below) while they are still available:
http://www.ebay.com/itm/190628670053?_t ... EBIDX%3AIT
The reason is that the filament on these is flat and about twice as "fat" as that on the bulbs usually available. In other words, the image of the filament fills the aperture of my highest NA dry objective (I didn't check the oil-immersion) whereas with the new bulbs I have been using, the length of the filament filled the aperture in the "y" direction, but the width was too narrow ("x" direction).
I have just received the 4 bulbs I bought from that seller and tried one and it appears to work very well.
[Disclaimer: I have no financial or other interest in the seller, nor do I know him/her, and this was the first time I bought anything from him as far as I know.]