MICROSCOPIC OBJECTS. 



9 



The use of artificial light has been con- 

 demned by many. One photo-micrographer 

 even goes so far as to say "artificial light 

 is a delusion;" but on comparing results we 

 shall find that, at least since the advent of 

 rapid dry plates, photo micrographs have 

 been taken by many workers fully equal to 

 any produced by daylight. In fact, certain 

 photo-micrographers who have most strong- 

 ly advocated the use of sunlight, have not 

 produced work even equal to that which 

 may be done by any manipulator of moder- 

 ate ability with artificial light after a few 

 months' practice. Artificial light is much 

 more easy to manage than daylight, and 

 does not vary so much in actinic quality; 

 hence exposures are easy to calculate, and 

 the illumination is more completely under 

 control. 



The light given by burning magnesium 

 is the richest in actinic rays. If the student 

 possesses a Solomon's magnesium lamp, he 

 will find no difficulty in working with this 

 light, the only objection to the lamp being 

 that it consumes the magnesium rather rap- 

 idly, and thus becomes too expensive to be 

 used constantly. It may, however, be used 

 with great advantage when photographing 

 very minute objects with high powers. The 

 most economical mode of using magnesium 

 ribbon is to burn it in a holder made of tin 

 or brass tube, the bore being just large 

 enough to admit the easy passage of the 

 ribbon. The tube should be about six 

 inches long, and mounted on a stand simi- 

 lar to that of the bull's eye condenser, with 

 joints to admit of proper adjustment. 



When using the holder, a spirit lamp 

 should be placed opposite the achromatic 

 or other condenser, and the magnesium 

 holder placed in such a position that when 

 the ribbon is thrust through the tube it 

 may enter the flame of the spirit lamp. 

 Some difficulty may be experienced in get- 

 ting the ribbon to properly illuminate the 

 screen, but a few experiments will render 

 the matter easy. The writer has made con- 

 siderable use of the above simple apparatus, 

 and very satisfactorily ; but as it necessi- 

 tates two manipulators — one to attend to 

 the light while focussing and arranging the 



correct position of the light, and another 

 to superintend the screen and focus — he 

 much prefers, for all purposes, a good paraf- 

 fine lamp. 



A well-made lamp is necessary, but good 

 paraffine still more so. Avoid all low- 

 priced, strong-smelling oils. Paraffine sold 

 at 8d. per gallon is not only unfit for pho- 

 to-micrography, but absolutely dangerous. 

 When burnt in any lamp with a large wick 

 it begins to evaporate rapidly as soon as the 

 lamp gets warm, and after a while the flame 

 will rush up the chimney, blackening it, per- 

 haps cracking it, and frightening the opera- 

 tor, if doing nothing worse. Reliable par- 

 affine may be had at is. 6d. per gallon, and 

 this will give more satisfaction in every way. 

 Duplex lamps may be used, but they pre- 

 sent no advantages for photo-micrography. 

 They give out much heat, consume a large 

 quantity of oil, and the double wick is 

 troublesome when using high powers. A 

 single wick is far preferable, but it should be 

 the broadest possible to obtain. 



The lamp devised by Mr. Dallinger spec- 

 ially for working with high powers, is, ptr- 

 haps, the very best yet made; but photo- 

 graphs can be taken with any paraffine 

 lamp. For some tim.e the writer used a 

 *iny microscope lamp, and took some very 

 successful photographs with lenses varying 

 from 2 inches to ^th inch, but the exposure 

 was necessarily long with all of them. 

 The object of the photo-micrographer 

 should be to make his exposures as short as 

 possible, and this can only be done by using 

 a powerful lamp and a vigorous developer. 



The light from a broad-wick paraffine 

 lamp will be found sufficient for even high 

 powers, but the brilliancy of the light may 

 be much increased by putting a lump of 

 camphor in the bowl of the-lamp. Gaslight 

 and candle-light are far loo unsteady to be 

 used for photo-micrography. 



Lesson IV. — Exposing the Plate. 



No rule can be laid down as to the dura- 

 tion of the exposure. It depends (ist) on 

 the focal length of the lens used ; (2nd) its 

 aperture, wide-angle lenses being far quick- 

 er than those of low angle ; (3rd) the na- 



