THE 



MONTHLY MICROSCOPICAL JOURNAL. 



APEIL 1, 1876. 



I. — Oil a New Arrangement for Illuminating and Centering 

 with High Powers. By Eev. W. H. Dallingbr, V.P.K.M.S. 



(Read before the Royal Microscopical Society, March 1, 1876.) 



Plate CXXXI. 



All who have been engaged in prolonged research, with the highest 

 powers at the present disposal of the microscopist, will have dis- 

 covered the practical value, not only of a delicate adjustment of the 

 illuminating jjencil, but also of an accurate incidence of the axis 

 of the sub-stage combination, employed as a condenser, with that of 

 the object-glass itself. The vahie of this was very amply demon- 

 strated to Dr. Drysdale and myself in our protracted employment 

 of high powers in our Researches on the Monads. 



On the subject of delicate centering, our best text-books say but 

 little, and for powers beyond the yVth the methods in common use 

 appear to me to fail, — at least in obtaining the delicacy of result 

 which we have found to be possible. 



Of course the theory of " centering " is that the optical axis of 

 the microscope should, by being prolonged, become accurately the 

 optical axis of the condenser. For this, in the first place, when high 

 powers are employed, there can be no "centering of the instrument" 

 in preparation for the successive reception of two or three difierent 

 powers. However delicate the workmanship, there is a displace- 

 ment in the transfer of powers, which, although of no moment in 

 the use of the ith, yoth, or rsth, is, in practice, when the best 

 results are sought, of the utmost moment when a change is made 

 from the yVth to the sVth, or from that to the ^^o^h. Each glass 

 must be centered for itself. 



For such centering, a simple cap pierced with a central hole of 

 about the one-twentieth of an inch, and placed upon the optical com- 

 bination of the condenser, is often employed. But it fails wholly to 

 accomplish the desired end ; while, as we shall presently see, the use 

 of a " centering glass " can only result in the accomplishment of 

 part of the end we know to be desirable, even if the apertures of the 

 diaj)hragms be made small enough for the highest powers. 



We now employ a very accurately centered diaphragm, under 

 the optical combination of the condenser ; this is pierced with great 

 care centrally, the aperture having a diameter of not more than the 



VOL. XV. N 



