84 DR. MATTHEWS ON A NEW METHOD OF ILLUMINATION. 



microscope, " That the manner in which an object is lighted is 

 second in importance only to the excellence of the glass through 

 which it is seen." This opinion I most heartily endorse, and that 

 literally, by adopting one of the objectives themselves, of lower 

 power, as an illuminator in place of a condenser, but not axially. 

 It must be so mounted as to send the whole of its bundle of rays 

 at angles varying with the requirements of any given case, and in 

 this consists the value of the method which I now introduce to 

 your notice. 



Any of the powers may be used, bearing in mind that the higher 

 the examining, the lower, within convenient limits, should be the 

 illuminating power, in order to secure a proportionate amount of 

 light. The only use of condensers of short foci and wide angle is 

 to get the more oblique rays of the cone by stopping out some 

 part of the rest. My best results have beeen procured by a two- 

 thirds or one-and-a-half object glass, which give in all cases quite 

 sufficient light. 



There is no limit for the angle at which the illuminator may be 

 used in relation to the axis of the instrument short of 90°, sup- 

 posing both the covering and the mounting glass as well as the 

 stage to be of no thickness ; but as they all have a very sensible 

 one, and it is found that rays of a greater angle than about 83° do 

 not pass through the slide to the object, but are reflected and 

 lost, I have found it better to work at angles varying from 25° to 

 65°. 



The objections which I have to urge against the use of very 

 oblique angles of illumination are as follows : — 1st. Supposing 

 thin covering and mounting glass to be used on wooden slips, the 

 shadows are likely to be so extended and long as to blend each 

 with the following, and so confuse the image. 2ndly. That if the 

 mounting slip be of the average thickness, it serves as a refracting 

 medium (like the earth's atmosphere during the setting of the sun), 

 into which rays of extreme obliquity enter ; but from which very 

 few emerge. And 3rdly, that in both cases much light is lost by the 

 dispersion of rays over a larger surface than would be covered by 

 the same number of rays at a lower angle. 



You will find a very useful and instructive paper by Mr. Hislop 

 on this subject in No. 3 of the " Club Journal," to which I refer 

 you. 



In relation to this part of the subject Mr. Ackland has made a 



