"Assuming that the power of the objective, the strength of the illumination and the angle of the beam converged on the object are in perfect correlation, the management of the substage diaphragm becomes important. If too widely open the image on the screen lacks contrast and it is hard to get a satisfactory focus. By gradually closing the diaphragm, which ought to be of the iris pattern, a point is reached affording marked contrast. To get the right focus it does not suffice to see the object generally well defined —its color becomes a potent aid to the average achro mat in order that it may exert its best actinic effect upon the sensitive plate. Remembering that the residual secondary spectrum is usually a palish green in most modern objectives, some delicate trans: parent detail of the object should be chosen, such as a fine bristle or beaded marking, and the micrometer adjustment be manipulated until a tinge is seen on the border line between green and rose. After this actinic focus is secured, I find it does good to still further slightly reduce the aperture of the diaphragm if the light be strong." " In photographing the interesting details of insects and vegetabe anatomy, three objectives are advisable, one of oen and-a•half or two inches focus, one of three-quarters, and a third of one-quarter. Being used without an eye - piece, prefer ence may be given to objected corrected for the American or English tube length rather than the short continental standard, for the lens is less strained when its conjugate focus is stretched to 30 inches. To these three may be added a good one-fifth or one-sixth if an achromatic substage condenser is to be used therewith. It is best to leave the higher immersion powers to the experts with their heliostats, lime-lights or 2,000-candle arcs at command, inasmuch as kerosene falls short of its duty in the premises, and even if a satisfactory focus can be obtained an hour's exposure is wearisome.
Subjects for successful photographing should be thin, level, and not too dark in color. Very thin sections are the best of all, but such delicate objects as the foot of a small spider, the feathery paddles of the tiny beetle-like creatures that affect stagnant pool sand have a habit of swimming upside down—the mouth parts of a mosquito, or the scales of a moth come up nicely. In photo-micrography there is practically no "depth of focus," so that thin ness and planeness are imperative. For my part I have taken a fancy to pathological horrors, and the Army Medical Museum has furnished me with many enticing sections of cancers, tumors and similar degenerated tissues. The editor of the "Annual" kindly reproduced a few of my pictures to illustrate a paper. One shows the liver of a mouse, invaded by a singular parasite, the trtcocephalus, which lays a multitude of eggs in the hepatic substance. Another gives an idea of the curious pulmonary com plaint, anthracosis, which attacks miners when their lungs become choked with the fine anthracite dust. A kindred disease is found among the dry grinders of Sheffield. For these two excellent preparations I am indebted to my good friend Dr. W. M. Gray. The view of the lances of the com mon blow-fly illustrates how critical lighting brings out the slender hairs.
I have dwelt so strongly on the imperativeness of securing this "critical" illumination if good results are aimed at, that it may be fitting to close this already too long paper with a com parison of the photo-micrography of a quarter of a century ago with that of our day,* taking that extremely difficult diatom, Antphzpleura pellucida, the markings of which are but 1.100 thousandth
of an inch apart, and so faint as only to be revealed by the most powerful objectives under the best conditions of oblique illuminations. The late Dr. Woodward was, I believe, the first observer to photo graph, in 187 T, this exasper ating test object which so f ew microscopists have really seen. Faulty light ing, apparently, has crowd ed the field with tion spectra. That these were not due to imperfec tion of the Powell & Lea land nth water immersion objective he used is infera ble from the clearness of the lines on the frustule proper. By the side of this old print is one taken last year by Dr. H. J. Ditmers, with a Queen -?..th oil im mersion lens, in which the vexatious ghosts are wholly exorcised. The striking dif ference seems mainly due to the critical improve ment in the achromatic condenser and in the more accurate regulation of the light rays.
The fascinating region of micrography deserves to be oftener explored by the amateur, and especially by those who, like myself, do not have much chance to work by day - light.
Dr. 0. G. Mason recently gave in the American Annual of Photography an interesting description of his apparatus, he says : Any person accustomed to manipulation with the compound microscope, even in some of its simpler forms, may—in these days of dry plates—by the expenditure of a few dollars, or—if handy, with tools, a little time—make such additions to his microscope as will enable him to do much valuable work. If he is a microscopist without any knowledge of the manipu lation and chemistry of photography, he yet has some of the most important elements of success, namely, the ability to properly light and focus his subject. The necessary caution in handling and timing exposure to light in the instrument is matter soon learned by experience, and the chemical work of developing and printing may be intrusted to some p'iotographer who will use care in his part of the work. The first attempts of the writer in this field of investigation were made with apparatus added to his microscope and at a cost of about six dollars. It consisted of telescoping box of wood and a holder for plates x 23% (the old ith size). The ocular end of the microscope body was closely fitted through the front end of this box, which rested on a proper support in the horizontal position, and offered very good facilities for coarse adjustment, while the fine adjust ment screw was with in reach of the ex tended arm when seated at the plate holder end of the in strument. Of course, under these conditions the amplification was not great, especially with low power ob jectives ; yet it was quite enough to make the work interesting, and wo uld well an swer for lantern slides, which is generally the ultimate use to which such photographic plates are put. As the work by such simple methods increases de sire for deeper insight in to the mysterious world of wonder which it reveals, more elabor ate methods are de vised for economizing time and reaching more perfect Such has been the prompting influence which led to the con struction of the instru ment here figured and described.