168 Transactions of the Royal Microscopical Society. 



these means the lamp and condenser have separate vertical motions. 

 But to accomplish the desired end we now want to be able to move 

 the whole pillar, lamp, and condenser, from side to side. To accom- 

 plish this the pillar is fixed to a solid piece of metal (Fig. 6), 

 which tits accurately into the grooves P, Q in the solid base of the 

 lamp T ; and by means of an endless screw motion the entire pillar 

 and lamp and condenser are carried horizontally to the right and 

 left, by the milled heads E, S, precisely as the mechanical stage of 

 a microscope is moved in the same directions. Thus a rectangular 

 motion of a delicate kind is given to the source of light ; and it is 

 easy to find the spot on the prism or mirror where the image of 

 the flame best secures the central illumination desired, and shown 

 in Fig. 5 ; for the illuminating apparatus can be worked while 

 the eyes are engaged with the microscope. Thus instead of the 

 uncertainty and weariness, often ending at last in failure, of pushing 

 the lamp from place to place, we can with comparative ease get the 

 central light we need, which as a rule I find it best to get with the 

 :^th first, and then in putting on the sVth or -sVth, a very slight 

 readjustment secures with these powers the same result. 



The drawing of the apparatus is made to a scale of one-fourth, 

 and the letters in each point to identical parts of the instrument. 



What the explanation of this illumination may be I do not 

 attempt to consider. The fact that this light can be got, the fact 

 that it can only be got with the utmost difficulty with an ordinary 

 lamp, and the fact of the immense value of such light in all 

 delicate biological investigations with high powers, every expert 

 microscopist may demonstrate for himself. 



The value of the method is not, however, confined to the use of 

 absolutely central rays, although it is in this that its supreme 

 value consists ; but it has a demonstrable value when we use larger 

 apertures, and the stops, for the resolution of test-objects. I have 

 invariably secured the finest results with the most difficult tests 

 by means of Powell and Lealand's supplementary stage and small 

 plano-convex condenser, which sends in the beam at as large an 

 angle as may be desired, only from one point ; therefore securing 

 shadows if they exist at all, or are within the compass of the power 

 used. But if the usual condenser * be centered and illuminated as 

 before described, and then the stops and apertures be carefully 

 employed, and the mirror gently manipulated, and at times the 

 lamp altered the minutest fraction, as experience will teach, the 

 very best attainable results may be secured. In this way I have 

 resolved with Powell and Lealand's new immersion ^th Frustulia 

 Saxonica,^ Amijhipleura pelhicida, Navicula rhomhoides (the dots 



* It may be •well to sny that it is Powell and Lealand's sub-stage condenser 

 that we have used throughout. 



t Two specimens from Germany and two specimens supplied by Mr. Wheeler ; 

 but only the transverse strife were seen. 



