Boss's Four-Tenths Condenser. 61 



a sufficient quantity of its oblique rays, when in focus, through, 

 an object seen with very moderate magnification, and should 

 be capable of giving a dark ground illumination with a half 

 inch or two-thirds object glass.* 



In Mr. Ross's four-tenths condenser, the principal desiderata 

 are excellently provided. The optical combination is exactly the 

 same as in his large angled four-tenths objectives, and the front 

 lens has a diameter of one-fifth of an inch. In the best con- 

 densers the diaphragms are brought close to the lower lens, and 

 this is the case with the instrument before us, which is pro- 

 vided with two revolving wheels of diaphragms. The upper 

 one is pierced with eight round holes, or " circular apertures," 

 as those who prefer learned phraseology may call them. One 

 of these is, in point of size, a copy of its next door neighbour, 

 and may be filled up with a polarizing shoe of tourmaline, or 

 may be made with a little rim, so as to receive any experimental 

 stops the microscopist may wish to try the effect of. Omitting 

 this stop, we have seven other stops, marked respectively 109°, 

 95°, 82°, 70°, 59°, 49°, and 40°. These stops allow the lenses 

 to work with the angles of aperture named. In the wheel of 

 diaphragms, below them, is another set of stops, which can 

 be combined with the preceding. A, the first, is a large 

 central stop adapted to 109° or 95° angles of aperture. B and 

 C, smaller ditto ; 3, 2, and 1 are radial stops — that is to say, 

 they are pierced with slots, three, two, and one respectively, 

 cut so as to converge towards the centre of the circle, but not 

 carried as far as the centre. The single slot stop keeps out the 

 central rays, and allows a radial beam, including its proportion 

 of marginal rays, to illuminate the object. This can be used 

 with the larger of the open stops. The two-slot stop gives 

 passages to two such pencils of light, one at right angles to 

 the other, a plan very effective with certain diatoms and other 

 objects. The three-slot stop allows the transmission of three 

 pencils, equidistant from each other. This gives the three 

 readings of the P. angulatum beautifully. The arrangement for 

 working the diaphragms in the condenser is very convenient, 

 and the whole apparatus rotates with Mr. Ross' substage. 

 The lenses are capable of adjustment to suit different thicknesses 

 of glass. 



Let us say a few words on the results. The proportion 

 which, from the size of the front lens the marginal and central 

 rays bear to each other is such, when the stop, allowing' 40° of 

 aperture is employed, that ' the two sets of lines, on Pleuro- 

 sigma hippocampus, and P. a.ngidatum, are distinctly shown 

 with a one-fifth objective and first eye-piece. A slight change in 



* With a two-thirds, we find it advisable to send the light through the 

 four-tenths condenser with the concave mirror, when a dark ground illumination 

 is required. 



