The time of exposure must be varied according to the intensity of the light, the sensibility of the Collodion, and the degree of magnifying power; a few seconds to a minute will be about the extremes; but minute directions are not required, as the operator, if a good Photographer, will easily ascertain the proper time for exposing (see page 224). At this point a difficulty will probably occur from the plane of the chemical focus not corresponding, as a rule, with that of the visual focus. This arises from the fact that the object-glasses of Microscopes are "over-corrected" for colour, in order to compensate for a little chromatic aberration in the eye-piece. The violet rays, in consequence of the over-correction, are projected _beyond_ the yellow, and hence the focus of chemical action is further from the glass than the visible image. The allowance may be made by shifting the sensitive plate, or, what amounts to the same thing, by removing the object-glass a little _away_ from the object with the fine adjustment screw; the latter is the most convenient. The exact distance must be determined by careful experiment for each glass; but it is greatest with the low powers, and decreases as they ascend. Mr. Shadbolt gives the following as a guide:--"An inch and a half objective of Smith and Beck's make required to be shifted 1/50th of an inch, or two turns of _their_ fine adjustment; a 2/3rds of an inch, 1/200th of an inch, or half a turn; and a 4/10ths of an inch, 1/1000th of an inch, or about two divisions of the adjustment. With the 1/4th and higher powers, the difference between the foci was so small as to be practically unimportant." There is also reason to think that the _kind of light_ employed has an influence upon the separation of the foci. Mr. Delves finds that with sunlight the difference between them is very small even with the low powers, and inappreciable with the higher; whereas in using diffused daylight which has undergone a previous reflection from white clouds, it is considerable. The object-glasses of the same maker, and particularly those of different makers, also vary much; so that it will be necessary to test each glass separately, and to register the allowance which is required. Having found the chemical focus, the principal difficulty has been overcome, and the remaining steps are the same in every respect as for ordinary Collodion Photographs. To those who cannot devote their time to Photography during the day, Mr. Shadbolt's observations on the use of artificial light may be of service. He employs _Camphine_, which gives a whiter flame than gas, or a moderator lamp; placing the source of light in the focus of a plano-convex lens of 2-1/2 to 3 inches diameter (the flat side towards the lamp), and condensing the parallel rays so obtained on the object, by a second lens of about 1-1/2-inch diameter and 3-inch focus.