DR. MATTHEWS ON A NEW METHOD OF ILLUMINATION. 85 



most valuable suggestion ; viz., to interpose a double concave lens 

 in the cone of rays formed by the objective condenser before it 

 reaches the object. There is no doubt that by these means much 

 light will be saved, since the rays will be rendered parallel, so that 

 fewer will be reflected by the under surface of the slide of that 

 part of the cone of light most obliquely incident on it. 



And now as to the results. There are, I think, gentlemen pre- 

 sent who can confirm me in the assertion that they have been most 

 gratifying and valuable. It was hardly to be expected that in 

 this — the infancy of the idea— with new manipulations to be as- 

 certained and mastered, new powers developed and perhaps new 

 appearances correctly interpreted, this method should exhibit its 

 superiority as speedily as it has done ; yet at even the very first 

 attempt I had the pleasure of resolving tests, which had been quite 

 impossible to me with the same powers previously. I will not 

 occupy your time by any attempt at enumeration of the objects 

 which I have thus observed and defined with unusual ease, for I 

 am of opinion that ocular demonstration is far better. I have, 

 therefore, placed a microscope on one of the tables, fitted up so as 

 to show my method, and Mr. Hislop has very kindly promised to 

 help me. We may not at first, while the management of the in- 

 strument is new to us, be able to show all that we could wish ; but 

 I quite believe that we shall, all of us, very soon be able to dis- 

 pense with other modes of using condensed light. One of its 

 happy developments is, s that we can at will, effect an excellent dark 

 ground illumination with objectives of almost any power or an- 

 gular aperture, by merely adjusting the illuminating power at a 

 slightly greater angle, easily determined by experiment. I 

 had hoped to be able to tell you that polariscope effects might 

 be obtained together with dark ground illumination ; but my in- 

 vestigations on that subject are as yet so far from complete that 

 I will not now lay them before you. 



The objectives which I have used are some by Beck, Ross, 

 Powell, and Lealand (an immersion of great excellence), Merz, 

 as well as one by Mr. Crouch of the same kind, with the per- 

 formance of which I was much pleased. You will be gratified 

 to hear that the cost of this useful addition to any microscope 

 need not exceed 25s., and that a substage is by no means ne- 

 cessary for its application, while the cost in those instruments not 

 having one need not exceed 15s. to 20s., and further that any 



