The Microscope. 7 



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

 error would probably overbalance the advantages of the reduced sec- 

 ondary 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 half inch down to three or four-inch, are now almost universally 

 constructed after the dialytic principle, being two widely-separated 

 systems, each consisting of a cover and flint glass of moderate optical 

 powers and fooming an achromatic lense, or nearly so, for itself. 

 This objective has almost perfect optical symmetry and forms, there- 

 fore, 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 newapochro- 

 matic 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 lense of my new 

 construction, adapted to the new glass. The | inch is a homogeneous? 

 immersion objective of 1.42 N. A., and -^^ inch 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 the eighth of 

 another new glass. The ^ inch 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 

 or two triplets, both of my new form. Thus these objectives are 

 made entirely of the new apochromatic glass. These new dialytic 

 objectives, aside from being practically entirely free from any dis- 

 turbing color, and in every other respect fully equal to the ordinary 

 dialytic of the best quality, are far supei'ior to any objective in flat- 

 ness of field, and are, therefore, unlike the European apochromatic 

 objectives, in less need of " compensating eyepieces " 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 



