ACCESSORY APPARATUS. 103 







the illuminating pencil, or to allow oblique light to pass only in some 

 one azimuth, or in two azimuths at right angles to each other. The 

 Achromatic Condenser of Messrs. Beck is a combination of three pairs, 

 of which the first and second are removable, so that the back pair may 

 be used alone for the illumination of objects viewed with low or medium 

 powers. The Achromatic Condenser of Messrs. Powell and Lealand has 

 an angular aperture of 170, and thus transmits rays of extreme obliquity 

 through objects mounted on thin glass; all other rays being excluded (if 

 desired) by a special arrangement of stops. The Diaphragm-plate being 

 perforated by apertures of different sizes, the largest of these (which 

 transmits the entire pencil) can be partially closed by centric or eccentric 

 stops attached to a separate arm, any one of which can be brought into 

 the optic axis; and thus, whilst the graduated apertures of the dia- 

 phragm-plate limit the peripheral portion of the pencil, the stops cut off 

 its central, allowing the transmission either of its entire peripheral por- 

 tion, or of the rays proceeding only from some special part or parts of it. 



Beck's New Achromatic Condenser. Webster's Condenser, fitted with Collins's 



Graduating Diaphragm. 



The same eminent makers have lately introduced a Non-achromatic Oil- 

 immersion Condenser; which, at a much lower cost, serves for the reso- 

 lution of the most difficult tests, their illumination by colored rays not 

 being found practically objectionable. In the Achromatic Condenser 

 now made by Messrs. Koss, extreme obliquity of the illuminating rays 

 is not provided for, this being obtained by means of their swinging 'tail- 

 piece' ( 72). Its combination has a focus of about 4-10ths inch; and 

 beneath its back-lens, which has an aperture of half an inch, is an Iris- 

 diaphragm for reducing it in any desired degree, with a rotating dia- 

 phragm-plate having a set of stops adapted to limit the aperture and to 

 give a ' black-ground ' illumination under objectives of different angular 

 apertures. Messrs. Beck have recently introduced a new Achromatic 

 Condenser with a front revolving eccentrically (Fig. 72), by which 

 means its focus maybe varied, and a ' black-ground ' illumination may 

 be obtained suitable for objectives having angles as high as 120. 



100 . Webster Condenser. Though the original idea of the arrangement 

 which has come into general use under this designation, and which is 

 at the same time comparatively inexpensive and applicable to a great 

 variety of purposes, was given by Mr. J. Webster ("Science Gossip," 

 April 1st, 1865), it has received important modifications at the hands of 

 the Opticians by whom the instrument is manufactured; and has, per- 

 liaps, not even yet undergone its full development. In its present form 



