158 N. E. GREEN ON A NEW STAGE ARRANGEMENT. 



the object, cutting off one or the other at pleasure by interposing a 

 piece of cardboard. Sti'ange indeed (to one unaccustomed to this 

 method of comparison) is the great alteration of appearance, as one 

 illumination is exchanged for the other. Delicate and beautiful 

 forms, which have just delighted the eye, and engaged the pencil, 

 brought into relief by the great obliquity of side-ray, vanish like a 

 dream, and are replaced by the old accustomed appearances with 

 transmitted light. So complete is the disappearance that we are 

 tempted to try an alteration of focus, hoping thus to recover 

 something of that which just previously had been so evident, but to 

 no purpose ; all effort is useless, till the transmitted ray is shut 

 off, and side-light again allowed to reproduce the lovely picture. 

 A few drawings of the surface markings of some of the Diatoniaceae 

 are exhibited in order that those accustomed to the usual appear- 

 ances may judge of the changes produced by thus varying the 

 method of illumination. 



We exhibit this evening, by means of the new stage, a slide of Angu • 

 latum, under a J^ by Zeiss, and would direct attention to the pearly 

 gray tone of the background, from which the valve is projected with a 

 brilliance like that of silver ; the advantage in this case being a 

 deliciously clear definition, without any glare of light upon the eye. 

 The brilliancy of the picture is doubtless due, to a great extent, 

 to the distance at which these German lenses focus from the 

 object, thus admitting the passage of a greater amount of light. 

 The definition of this lens by Zeiss must speak for itself. Tri- 

 ceratium, under a JL with the lime-light, and a valve of Isthmia 

 are also exhibited as examples cf the results of side-illumination. 



Plate xiii (lower half), illustrates this paper. 



