94 



PHOTO-MICROGRAPHY 



photographed around each little object, which will quite spoil the final result. Then 

 again, when using what is called "oblique light" — that is to say, when a diaphragm 

 is placed eccentrically in the base of the condenser, so as to limit the light entering it 

 to one small edge of the back lens, thus passing obliquely on to the object — then, too, 

 is the higher aperture of the condenser to which we have referred of definite use. It 

 is obvious here why the extra aperture should be of service, as it necessarily permits 

 a greater degree of obliquity of the rays passing to the object than would be per- 

 mitted by one of smaller N. A. The application of oblique light as applied to photo- 

 micrography will be given in a future place when dealing with objects which require 

 its especial use, but mention is merely made to it here as connected with what we 

 think is the real value of these condensers of high N. A.; that is, until objectives are 

 made which will stand their use. 



With respect to the hollow cones of light and their utility, much diflference of 

 opinion also exists. For testing and showing the inferiority of the outer zone of all 

 objectives, even the very best of apochromatics, this annular light may be most 

 advantageously employed by the optician, but we confess to agree with many who 

 are more qualified to give an opinion than ourselves, that as to its increasing defini- 

 tion — except, perhaps, on the vexed question of how many lines can be resolved to the 

 inch with a given aperture — is altogether a very doubtful and debatable matter. It 

 is true those who are its advocates reply that it is only oblique light in all azimuths, 

 and that is so ; but to the use, except under special circumstances, of oblique light at 

 all we beg to demur, for it is not difficult to show that it is a most dangerous experi- 

 ment, filling the image with optical phenomena perhaps in no ivay connected with the 

 real details of the object itself. It is, moreover, evident from the studies of Professor 

 Abbe in recent years, that the final image afforded by the best of extremely high 

 power objectives that can be made is not to be regarded as an absolutely faithful 

 representation of the minutest details of the object in their final form, unless, indeed, 

 the objective has a N. A. of about 2"oo, and is used in conjunction with a suitable 

 substage condenser and with an immersion fluid of sufficiently high a refractive 

 index, all of which at present seem to be absolutely unattainable. Therefore, it 

 is of very doubtfid advantage to imperil the close approximation to the truth we 

 already possess in the performance of our highest power objectives by adding to 

 the^final image produced by them a series of the minutest details, which in all 

 cases may, and in many positively are, due to optical phenomena of the highest 

 order of complexity — false images rightly called, indeed, and which are only of 

 interest to the optical student, and which can be varied at pleasure. If extreme 



