ROSS'S CONDENSER. 167 



seen with very moderate magnification, and should be 

 capable of giving a dark-ground illumination with a J 

 inch or ds object glass. 



In Mr. Ross's j^ths condenser, represented at A, 

 fig. 39, these desiderata are well provided for. The 

 optical combination is exactly the same as in his large 

 angled -f^ths objectives, and the front lens has a diarMeter 

 of. th of an inch. In all good condensers the diaphragm 

 should be brought close to the lower lens ; that is the 

 oase with this instrument, which is provided with two 

 revolving wheels of diaphragms. The upper one is pierced 

 with eight circular apertures ; one of which is filled in 

 with a polarizing slice of tourmaline ; another is made 

 with a little rim, so as to receive any experimental stops 

 the microscopist may wish to try. Omitting this last- 

 mentioned stop, we have seven others, marked respec- 

 tively 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 

 arranged another set of stops, for combination with the 

 preceding. 



A 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. The arrangement 

 for working the diaphragms in the condenser is very con- 

 venient, and the whole apparatus rotates with the sub-stage. 

 The lenses are also capable of adjustment to suit different 

 thicknesses of glass. 



The proportion which, from the size of the front lens, 

 the marginal and central rays bear to each other, is such 

 that, when the stop allowing 40 of aperture is employed, 

 the two sets of lines, on Pleurosigma hippocampus and P. 

 angulatum, are distinctly shown with a one-fiftn objective 

 and an A eye-piece. A slight change in the position of 

 the flat mirror makes this stop work excellently with the 



