286 SUMMARY OF CURRENT RESEARCHES RELATING TO 



respect, but with the- secondary spectrum considerably reduced, and, 

 consequently, the definition greatly improved. But, unfortunately, this 

 is not the case. In my paper at last year's meeting, I pointed out the 

 tact, derived from figures of the refractive and dispersive powers of the 

 new glass, as furnished by the makers, that the proportions of powers 

 were such as to require extremely short curvatures, which would produce 

 a very injurious amount of aberrations of the second order, and that this 

 error would probably overbalance the advantages of the reduced secondary 

 spectrum. Since that time, however, I have tried the glass, and found 

 my assertion to be correct. Indeed, it could not well be otherwise, as 

 figures seldom lie. In fact, it would be impossible to construct from 

 the new glass Microscope objectives of superior quality after the usual 

 or known plans. Our present low powers, for instance, from 1/2 in. 

 down to 3 or 4 in., are now almost universally constructed after tho 

 dialytic principle, being two widely-separated systems, each consisting 

 of a crown and flint glass of moderate optical powers and forming an 

 achromatic lens, or nearly so, for itself. This objective has almost 

 perfect optical symmetry, and forms, therefore, a very even and flat field 

 of fine definition and brilliancy. No addition of lenses, nor any change 

 of form could improve this objective, but would rather impair its quality. 

 But the new apochromatic glass is entirely unfit for this form of objective, 

 for the reasons heretofore given. I was led, therefore, to consider 

 whether another form of construction could be found to which the new 

 glass could be advantageously adapted, and I have succeeded in solving 

 the problem so completely that, for theoretical reasons, I do not hesitate 

 to claim my new formula to be the only proper one for the new glass. 

 My new apochromatic objectives contain at least one triple lens of my 

 new construction, adapted to the new glass. The 1/8 in. is a homo- 

 geneous-immersion objective of 1 • 42 N. A., and 1/50 in. working distance. 

 It contains two triple systems and two single lenses, of which the back 

 system is constructed after my new invention. Of this objective seven 

 lenses are made of the new apochromatic, and tho eighth of another new 

 glass. The 1/4 in. is a dry working objective of 100° aperture. It is a 

 three-system, and all but one of its lenses are made of the apochromatic 

 glass, the back system being a triplet of my new form. The low powers 

 are constructed after the dialytic, and consist of two triplets, both of my 

 new form. Thus these objectives are made entirely of the new apo- 

 chromatic glass. These new dialytic objectives, aside from being 

 practically entirely free from any disturbing colour, and in every other 

 respect fully equal to the ordinary dialytic of the best quality, are far 

 superior to any objective in flatness of field, and are therefore, unlike 

 the European apochromatic objectives, in less need of ' compensating 

 eye-pieces ' than the best ordinary objectives. 



As a very important advantage of the new apochromatic objective 

 over the ordinary one, I regard the absence of a separate chemical focus, 

 which quality makes the objective especially adapted to photographic 

 work. A 1 in. has recently been tested photographically, with a distance 

 of 1^ feet between the objective and the image, and not a trace of the 

 usual difference between the visual and active foci could be found, and 

 the resulting picture was of unusual sharpness and brilliancy." 



Dr. F. L. James says * that " one immense advantage which these 



* St. Louis Med. and Surg. Journ., liii. (1S87) pp. 356-7. 



