WEBSTER CONDENSER. 



Ill 



angle of convergence, and have the effect of those transmitted 

 through the peripheral portion of the ordinary Achromatic Con- 

 denser. When, on the other hand, this combination is used with 

 a diaphragm that allows only the central rays to pass, these rays 

 meet at a small angle ; and the illumination thus given is very 

 suitable for objects viewed with low powers. Again, by stopping 

 out the central portion of the combination, and removing the Con- 

 denser to a short distance beneath the object, the effect of a Black 

 ground illumination (§84) can be very satisfactorily obtained with 

 Objectives of moderate angular aperture. Further, by stopping- 

 out not only the central but also a great part of the peripheral 

 rays, so as only to allow the light to enter from a small portion or 

 portions of the margin, oblique illumination (§ 81) can be most 

 effectively obtained. All this can be provided for by a Diaphragm- 

 plate made to rotate at as short a distance as possible beneath the 

 condensing-lens ; but as the number of apertures in this plate is 

 necessarily limited, a greater variety is obtained by the use of a 

 Graduating Diaphragm (§ 78) for the regulation of the central 

 aperture, and by making the apertures in the rotating plate sub- 

 servient to the other purposes already named, as is done in the 

 arrangement of Mr. Highley (who employs the Dollond Diaphragm) 

 and Mr. Collins (Fig. 61). — Still greater variety can be obtained 

 by means of another 

 very simple arrange- 

 ment more recently 

 introduced by Mr. Col- 

 lins ; the tube which 

 carries the lenses being 

 fitted with another 

 tube which slides with- 

 in it ; and the summit 

 of this last being fur- 

 nished with a socket 

 into which may be 

 inserted a diaphragm 

 of blackened card or 

 of thin metal, with an 

 aperture or apertures 

 of any shape or size 

 that may be desired. 



Fig. 61. 



Webster's Condenser, fitted with Collins's 

 Graduating Diaphragm. 



In this manner the Diaphragm may be carried up quite close to the 

 Condensing lens, which is a great advantage ; and when Oblique 

 illumination is desired, the light may be transmitted from any 

 direction by simply giving rotation to the tube carrying a diaphragm 

 with a marginal aperture. — The Webster Condenser thus improved 

 (which may also be used in combination with the Polariscope) 

 will be found one of the most universally-useful accessories with 

 which a Student's Microscope can be provided. 



