Notes. 297 



years he alone made condensers suitable for the highest kinds of 

 critical work. The following short account contains the history of the 

 achromatic condenser. In 1838 an achromatic objective was used as 

 a condenser by Dujardin ; in 1839 this plan was adopted by Andrew 

 Boss. The achromatic condensers we find in 1840 are merely the 

 achromatic quarters of that period having an aperture not greater 

 than 0*35 N.A. ; these had no stops or diaphragms of any kind. 

 In 1849 Gillett's condenser was introduced ; its aperture was about 

 0*65 N.A., and it had a rotating wheel of stops. In 1854 Powell 

 brought out a new achromatic condenser of 0*76 N.A., which, like 

 Gillett's, was provided with a wheel of stops. In 1859 he improved it 

 by increasing its aperture to ' 98 N.A. ; but it is stated that in order 

 to utilise its full effect, it is necessary to have the object mounted 

 between two cover-glasses. This defect must have been subsequently 

 remedied, as I possessed one of these condensers and found that it 

 would work through any ordinary slip. The formula of this 1859 

 condenser was altered somewhere about the end of the seventies, its 

 aplanatic aperture being increased, otherwise it remained the same. 

 This new formula condenser may be distinguished from the older one 

 by the piano front of its second lens, the second lens of the other 

 having a concave front surface. This 1859 condenser remained 

 unapproached for critical microscopical work, until the substage con- 

 denser was apochromatised by Powell in 1895. In April 1869, 

 Messrs. Powell and Lealand brought out a side bull's-eye, which, 

 used in conjunction with a super-stage, gave a powerful beam of very 

 oblique illumination. It is probable that it was with this instrument 

 that the so-called " striae " on Amjohipleura pellucida were really first 

 resolved. 



About the year ] 870 Powell brought out a new arrangement of 

 prisms for obtaining non-stereoscopic binocular vision, with high 

 powers. The utility of such appliances is doubtful, because they all 

 more or less impair the sharpness of the image ; the only thing that 

 has ever been said in their favour is that they are less fatiguing than 

 the monocular, but it is doubtful if they possess even this one 

 advantage. 



Hugh Powell died November 1883. An excellent portrait of him 

 was presented to our readers as a Frontispiece to the volume of this 

 Journal for 1899. 



