DR. MATTHEWS ON A NEW METHOD OF ILLUMINATION. 81 



equally at all angles, and notably least at the most oblique 

 — a circumstance fatal in practice — and that they impart colour 

 to otherwise colourless objects, either by refraction or probably 

 some amount of polarization and perhaps diffraction. 



" Nachet's " prism — in which light is taken from the plane mirror 

 (but not necessarily so), conveyed by two internal reflections to its 

 apex, which is surmounted by a plano-convex lens, and thus con- 

 verged on the object — is another form of great merit, for it con- 

 denses oblique rays and is so mounted as to throw them in any 

 azimuth by a revolving fitting. But, as its angle is unvarying, it 

 has not been attended by the advantages expected. 



It was speedily found that in none of these methods is sufficient 

 light reflected in the use of the higher powers, so that it became 

 necessary to devise some means of concentrating more light upon 

 certain objects under certain requirements. Out of this necessity 

 grew a new, complicated, and expensive instrument, called a con- 

 denser. This certainly fulfilled its purpose well, too well in fact, 

 for in the blaze of light thus collected, nearly, if not quite, all de- 

 finition was lost. This speedily demanded and obtained a remedy, 

 though it was, as I shall presently endeavour to show, of the most 

 objectionable kind. And here arises an involuntary sigh of regret 

 that so much ingenuity of arrangement, such delicacy of construc- 

 tion as is displayed in the condensers of our best English micros- 

 cope makers, should be expended in such a wrong cause, in such 

 an erroneous direction, while, as is too often the case, the remedy 

 lay at our very feet, unnoticed and neglected ! 



Condensers are of two kinds, though similar in principle and 

 but slightly varying in construction. 



Firstly. One or more lenses interposed between the mirror and 

 the object, not achromatic. Of this kind Mr. Reade's " Kettle 

 Drum " is a good illustration, and being reasonable in cost and not 

 difficult of use, has been found very serviceable. 



Secondly. One or more lenses, also interposed between the mirror 

 and the object, but perfectly corrected in all respects in the manner 

 of the best objectives. Of this form the admirable instruments of 

 Messrs. Beck, Ross, Powell and Lealand, Baker, Crouch, and 

 others are good examples, as well as, though last not least, the 

 excellent arrangement of Webster, on and by which most of my 

 earlier experiments were made. It is very efficient, besides being the 

 simplest and cheapest form ; no small consideration, I surmise, to 



