63 Reade, on a New Hemispherical Condenser. 



tion for illuminating any rectangularly marked valve whose 

 position on the stage of the microscope is known. One 

 point of light must lie over the end of the valve for bringing 

 out the horizontal lines; the other will be opposite the 

 side of the valve, and will act on the longitudinal lines ; and 

 resolution into dots or squares will be immediately eflfected 

 by adjusting the distance of the condenser. 



For oblique or diagonal markings, the apertures at the cir- 

 cumference of the diaphragm must no longer be placed at 90'' 

 apart, but at such an angle as is indicated by the markings 

 themselves. In the case of the P. migulatum, where there are 

 three lines of markings, there must be three apertures, 

 since with two apertures only, we should exhibit, according 

 to their position, any two, and but two, of these three lines, in 

 turn, and, at the same time, give a sort of unnatural elongation 

 to the peculiar markings on the valve. The size of the 

 apertures is 24° at the circumference and opposite side, and 

 T^ths of an inch in the direction of the radius. The 

 latter dimension must be less in diaphragms for smaller 

 hemispheres, and must never exceed half the radius of the 

 condenser. 



In order to secure the best effect, the distance between 

 the apertures must be adjusted with considerable accuracy. 

 For this purpose I use a diaphragm of thin brass, or of strong 

 tinfoil, having one aperture only, and by its rotation under a 

 given valve of the P. angulatum, for instance, I bring into 

 view the three lines of markings in succession, first the 

 horizontal lines, and then the oblique lines, by rotating 

 the diaphragm to the right and left, and thus the three 

 points at which the apertures are to be made can be deter- 

 mined with the utmost precision. If the aperture for the 

 horizontal lines be made at the distance of 180° from the 

 place thus obtained, these lines will be illuminated on their 

 opposite sides, and the three apertures will be 120° apart, as 

 in the diaphragm first cut out for me by Mr. Water house, 

 who happened to be working with me at the time. But 

 in practice I find it not only better, but indispensable, to 

 illuminate all the markings on the same side, as by the first 

 method, and preserve thereby that uniform direction of the 

 shadows which is the key to accurate definition. A set of 

 diaphragms thus obtained, and a diaphragm with a minute 

 circular aperture in the centre only, for the central adjustment 

 of the lens, complete the furniture of the condenser ; and a 

 brass ring sliding outside the top of the cylinder on which the 

 condenser is screwed conveniently holds the diaphragms in 

 their place, and admits of their being readily changed. 



