Fig. 14.
It consists essentially in allowing the light to fall upon the object from below, at such an angle that none of it can enter the objective directly. Thus the concave mirror, turned as far as possible to one side, and reflecting on to the object the rays from the lamp placed upon the opposite side, will give very fair results with low powers; this plan, however, is capable of but very limited application. Again, a disc of black paper may be stuck on to the middle of the bull’s-eye, and the latter be placed below the stage between it and the mirror. In this case everything depends upon the size of the disc, which, if too small, will not give a black ground, and if too large will cut off all light from the object.
The best and only really satisfactory plan is to arrange the illumination with the substage condenser, as previously described, and then to place below the lens of the latter a central stop of a suitable size, which can only be determined by trial. When this has been done the object will be seen brilliantly illuminated upon a field of velvety blackness. Such stops are supplied with the condenser.
We have devoted a considerable portion of space to this question, since it is, of all others, the most important to a successful, satisfactory, and reliable manipulation of the microscope; but even now, only the main points of the subject have been touched upon, and the worker will find it necessary to supplement the information given by actual experiment. A few failures, rightly considered, will afford a great amount of information, but those who desire to go thoroughly into the matter are recommended to consult the present writer’s Guide to the Science of Photomicrography, where it is treated at much greater length, as an essential part of the subject-matter of the book.
It may be added here, that no method of reproducing the images of objects is on the whole so satisfactory as the photographic one; and whilst a lengthened reference to the topic would be out of place in a work of the character of the present one, the one just mentioned will be found to contain all that is necessary to enable the beginner to produce results which, for faithfulness and beauty, far excel any drawing, whilst they have the additional advantage that they can, if required, be exhibited to hundreds simultaneously.