ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 521 



CareM manipulators of the Microscope have long felt the damage 

 to the sight which was involved in the attempt to gaze at objects in a 

 full glare of light, and so it came to pass that ' Iris ' and other dia- 

 phragms were introduced below the condenser to allow only a very- 

 small pencil of light to be transmitted ; but there was a consciousness 

 that the device was, in degree, tantamount to weakening the light, 

 and it has always been an axiom iu microscopy that ' a weak lif'ht 

 will never give a brilliant image ' ; and then came an attempt at 

 additional intensity through a lamp with a flat flame. For resolution 

 this answered admirably, and necessary obliquity obviated glare, but 

 with axial illumination this intensity was not easy to deal with. If 

 the ' Iris ' was used to moderate light to a small aperture, lines were 

 thickened, and images of objects looked coarse ; if the diaphragm 

 was not freely used the image was poor, flat, and milky, and drowned 

 in a flood of radiance. So diaphragms were constructed, as ' Calotte,' 

 &c., which would act above the condenser, and so, when the full 

 body of light was focussed on the object, the aperture in the con- 

 denser would suppress such a number of rays as went rather to injure 

 the image. 



There is a positive value in such appliances, and provided they 

 are made sufficiently thin, so as to be easily revolved and easily 

 got at for manipulation, they will do all, or very nearly all, I 

 claim to get from the small and simple contrivance I am about 

 to describe. But it is not to be lost sight of, that these special 

 forms of revolving diaphragms applied above the condenser must 

 of necessity be costly, and confined almost exclusively to what are 

 classed as ' expensive Microscopes,' and it is no small satisfaction 

 to be able to say that experiments have shown me that equal or even 

 superior effects can be got with appliances costing but a few pence, 

 and requiring little more skill in manipulation than a fair amount of 

 delicacy of touch. We recognize that the conditions to be fulfilled 

 are to interpose some screen between condenser and glass slip carry- 

 ing the object, which shall work easily in the very small space be- 

 tween the usual so-called -^^y condenser and the said slip, and which 

 can be made to cut oif gradually from the blaze of light all the rays 

 not actually wanted for the perfect illumination of the object imder 

 inspection, and at the same time, that such a screen shall be as simj)le 

 and as little costly as may be compatible with thorough efficiency. It 

 will be seen how far these conditions are met. It would be useless to 

 detail the hundreds of forms of screens or diaphragms experimented on, 

 ■ — all imaginable shapes of aperture and of all sizes, squares, triangles, 

 slits, slots, ' cat's-eyes,' bars, and central stops to apertures, and all 

 more or less unsatisfactory in some way ; better to narrate the settling 

 down into the conviction that there really was great advantage in the 

 employment of the simple means shown in the accompanying diagram, 

 where all is given in actual sizes used. It will be seen that there are 

 three slips, of either thin smooth card or vellum, each 4 inches by a 

 full f inch, and in each card are punched a row of apertures oblique to 

 the long axis of the card, and consequently to the slip of glass when 



