1893.] MICROSCOPICAL JOURNAL. 349 



cal selenite stage, in a compact and desirable position, between 

 the condenser above and the polarizer below, all in close con- 

 junction. For other things, more than can be enumerated 

 now, workers will find the space between the upper and lower 

 holders of this sub-stage both available and useful. 



To sum up shortly, it seems to me that a sub-stage so ar- 

 ranged and constructed offers several advantages ; it does away 

 with swinging out to one side the iris diaphragm for any pur- 

 pose, thereby putting extra weight and leverage on that side and 

 incurring strain to working parts ; it preserves a constant bal- 

 ance of all weights about the lines of motion ; it allows conven- 

 ient removal and replacement of the diaphragm ; it permits the 

 substitution of other apparatus instead of the diaphragm ; it can 

 be exactly centered in the optic axis ; it is simple, efficient aiid 

 inexpensive. 



I should note, perhaps, that in the above description I have 

 had in mind the iris diaphragm as made by Zeiss and sold sep- 

 arately ; I have also shown in the drawing the Abbe condenser 

 system, as furnished in its mounts by Zeiss, fitted into a 

 jacket entering the holder from above instead of below as is 

 customary with that maker. 



The only objections that I imagine can be raised to this sub- 

 stage are that the iris diaphragm is not in closer contact with 

 the condenser, and that the manipulation of the inserted appara- 

 tus is delicate. The first objection I consider theoretical rather 

 than practical as actual measurements will demonstrate that a 

 construction which contemplates swinging aside the carrier 

 and diaphragm must allow sufficient space for free passage, and 

 the operation of withdrawing and replacing the diaphragm as 

 described requires scarcely, if any, more space than usually al- 

 lowed for the former operation. The second objection is sub- 

 jective and probably can only be cured by education. There 

 will always be people who use sledge hammers to kill mosqui- 

 toes, just as there will always be microscopists who want the 

 stages of their 'scopes large and strong enough to serve as plat- 

 forms on which to stand and deliver dissertations on its con- 

 struction. Nevertheless to all interested I submit this modified 

 sub-stage for trial and judgment in the belief that fulness of per- 

 fection is attained in time onh' by aid of attempted individual 

 contributions, good, bad and indifferent, to that end. 



