The Lantern Microscope. Bij Leivis Wright. 203 



plainly visible through an opera-glass. This is indeed the real 

 difficulty. 



But, first, such an instrument much facilitates photo-micro- 

 graphy, beautiful images being obtainable with facility, and with 

 no other apparatus. Whether or not diatoms may be sufficiently 

 shown by the lime-light on the screen for an audience to see the 

 details, lovely images are easily obtained, either direct or with 

 black-ground illumination (up to 12 in. in the one case or 

 6 in. in the other), of such brilliancy that very short exposures 

 amply suffice. As any condenser or lens found to do good work 

 can be employed, with no preparation whatever beyond stopping 

 stray light by black cloth, I hope there may be some field for 

 usefulness in this direction. 



The other direction is, the improvement of lenses as regards 

 their aberrations from flatness of field. My experiments have 

 shown how far most lenses are from perfection in this respect. 

 Now a moment's reflection will suggest, that if a lens called " flat " 

 over all the field of the compound Microscope, breaks down utterly 

 in flatness over a field three times the diameter, it is not really 

 flat at all. If we are to express the deviation from flatness by a 

 trace drawn to ordinates in the familiar way, it is for instance 



Fig. 41, Fig. 42. 



utterly impossible that the field can be really flat from A to B as 

 in fig. 41, and the aberration commence and rapidly increase from 

 A to C and B to D. The whole must be a curve resembling C A B D 

 in fig. 42, A to B being only approximately flat but not really so. 

 And this is the case ; for it will be found that if such a lens is 

 focused for the centre of the field, the microscopist, for his own 

 comfort, brings the point under examination to the centre. This is 

 particularly the case with half-inch powers. But with the one 

 modern half-inch lens which, as already stated, I found to " hold 

 out " to the edge of the screen field, it is not so ; the eye rests with 

 equal comfort on any part of the field of view in the compound. 1 he 

 lest lens on the screen is the best on the compound, as regards this 

 particular point. It is perfectly true, that if a lens be worked out 

 solely for the screen, it may prove very second-class for compound 

 work ; for there are other important elements which this test does 

 not touch. But if the lens is properly computed for these other 

 elements of general work, and its corrections for flatness are worked 

 out on the screen, then I Lave every reason to believe that a far 

 better result will be obtained. I only know one optician, to whom 



