1883.] 



MICROSCOPICAL JOURNAL. 



8 = 



excellent results. We would as will- 

 ingly use one brand as another ; for 

 the differences in sensitiveness and in 

 development are of little consequence, 

 if one works intelligently in the dark 

 room. We would advise (he use of 

 slow-working plates, however, when- 

 ever great rapidity is not absolutely 

 required. The demand for rapid 

 plates has led manufacturers to vie 

 with each other in producing the most 

 sensitive plates possible. These are 

 mainly called for by amateurs in field 

 photography, and for in-door work in 

 studios. 



In passing to such extremes the 

 art in the work is sacrificed. To ob- 

 tain fine gradations of light and shade, 

 time must be given to the plate in the 

 camera. For this reason we have 

 preferred slow plates for landscapes, 

 and never would use a quick plate ex- 

 cept for instantaneous views. For 

 photomicrographs we have used the 

 " B " plates of Carbutt. 



Exposure. — The exposure should 

 be just long enough to give all possible 

 detail in the shadows, and at the same 

 time long enough to give density in the 

 high-lights. To the novice this sen- 

 tence will require some explanation, 

 and it is essential that it be fully un- 

 derstood if one would work intelli- 

 genth'. To explain it we will suppose 

 the subject is a landscape, because 

 we can the more readily use such an 

 example than a microscopic specimen 

 which might not be familiar to every 

 reader. 



In a landscape, the sky is the bright- 

 est object of all. The sky should 

 print white, hence on the negative, 

 where the lights and shadows arf' re- 

 versed, the sky must be blaek. The 

 intensity of the negative representing 

 the bright parts of the object will de- 

 pend upon the exposure that has been 

 given. But the strong light of the 

 sky impresses the sensitive plate very 

 quickly and strongly, and a very short 

 exposure gives an opaque sky. 



But when we consider the shadows 

 and dark objects in the landscape, it 

 will be seen that there are gradations 



of light and shade, all of which must 

 be impressed upon the plate. In 

 order to get them the exposure must 

 be so timed that the comparatively 

 weak light reflected from such objects 

 can act upon it. Obviously this re- 

 quires a longer time than the high- 

 lights, hence when the dark parts are 

 properly exposed, the sky is over- 

 exposed. For this reason, the clouds 

 do not show in a photograph, — the 

 entire sky prints perfectly clear, no 

 matter how many clouds were visible 

 when it was taken. From this it will 

 be correc'.ly inferred that the exposure 

 must be timed to correspond to the 

 dark parts of an object, regardless of 

 the rest ; for the details, either of 

 light and shadow, or of texture or 

 structure, must be brought out in the 

 dark portions. 



This is especially necessary to con- 

 sider in microscopical work. We have 

 seen many photographs of microscopic 

 objects which showed the outlines 

 distinctly, but in which detail was 

 quite wanting. Take almost any 

 preparation of a large insect — the 

 proboscis of a bee, or the sting, for 

 example — and make several photo- 

 graphs with different exposures, and 

 the results will afford a good idea of 

 the time necessary for the yellow and 

 reddish parts to be properly impressed 

 on the plate. 



Development. — After the plate 

 has been exposed the image is invisi- 

 ble, and has to be brought out by the 

 developer. The composition of the 

 developer will be given further 

 on. Suffice it here to say that the 

 developer causes all parts of the 

 plates which have been acted upon 

 by light to turn black, leaving the 

 rest white. It is during the process 

 of development that the operator 

 learns whether the exposure has been 

 about right or not. If the image 

 flashes out quickly, the plate has been 

 over-exposed ; if it develops very 

 slowly, under-exposed. The books 

 say that a negative from an over- 

 exposed plate is weak, and lacks con- 

 trast, while an under-exposed plate 



