522 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the card as intended is placed beneath the slip upon the stage of the 

 MicroscoiJe. It will be seen from construction that on any stage, and 

 with the object to be looked at mounted even excentrically, there 

 would be no sort of difficulty, when the slip is duly clamped and the 

 object brought to centre with a low-power objective, in moving the 

 perforated ' stage diaphragm ' right or left so as to let the object be 

 in the centre of a circle of light. With any power used, it is easy to 

 understand tliat a slight push or pull at one of the free ends of the 

 card will, without disturbing the object in the slightest degree, cut 

 off more and more f)f the unnecessary rays, and as these are gradually 

 eliminated, it will be found that the image changes from a poor, flat, 



drowned effect to one of great 

 *^'^' ^^■^" boldness and brilliancy. It is 



here that the delicacy of touch 

 comes in, for the finest effects of 

 detail are often got with an all 

 but imperceptible movement of 

 the card. One may say that the 

 card material lends itself better 

 to this delicacy than perhaps 

 metal would do, inasmuch as 

 the small amount of friction 

 between slip and card and card 

 and stage resjJectively, keeps 

 this simple and efficient dia- 

 phragm in its place, and it is 

 evident that if card or vellum 

 be the material employed it will 

 have the additional advantage 

 of not scratching the condensing lens. Three apertures of diaphragm 

 were found available, ^, ^, and ^, all with certain advantages with 

 different powers and different classes of objects. 



It was both interesting and useful to try against this stage 

 diajihragm the effect of corresponding apertures in so many " caps " 

 to the condenser. The circumstances would of course be a little 

 different, inasmuch as with cap to condenser in order to modify the 

 light the said condenser would have to be moved bodily, and the 

 effects would be got by a quasi marginal instead of a central pencil, 

 and so be analogous to some of the effects of oblique light — a con- 

 dition not at all desirable except for ' resolution' of very diaphanous 

 objects. Anyhow, aperture for aperture, experiment demonstrated 

 that with delicate touches far finer effects could be got from the stage 

 diaphragm than from any sized cap on the condenser, perchance from 

 the light being absolutely central to begin with. One can under- 

 stand that many conditions would have to be considered in order 

 to determine the point, but I have thought it best to confine myself 

 to practical points. One practical point soon made itself evident, 

 and that was, that the aperture either in card or caj) could not be 

 reduced beyond a certain size with advantage. This limit was 



