DIAGRAM EXPLAINED. 167 large one, and the image of the object to be viewed being formed between these two lenses, the rays proceeding from it are made again to converge, on passing through the upper lens or eye-glass, in the focus of which, a distinct and greatly magnified image of the object is seen. I hope this drawing will make the affair still plainer. The first figure is a microscope, like this we have been examining: the second is a diagram, in which slanting lines show the course of the rays, and the refractions they sustain in passing through the different lenses. The arrow a b, represents any small object, which you may suppose to be laid on the stage of the microscope, as at F. C is the objectglass, such as you have seen screwed on the instrument at the part marked O in the drawing. The rays proceeding from the object, on entering this small lens, are converged. They cross one another, and pass out of the lens diverging: if they met with no interruption, these rays would continue to rise up through the body of the microscope, and would form, in the upper part 168 DIAGRAM EXPLAINED. of it, an image, ST, so large, that only a part of it could be seen at once, by an eye looking in through a lens fixed in the top of the instrument. By using two eye-glasses we obtain as large an image, and see the whole of it. They act in this way :— The first eye-glass, AB, receives the diverging rays, and refracts them, bringing them to a focus at a shorter distance than ST; an image is accordingly formed there, smaller than ST. The rays, which had been made to converge by passing through the lens AB, go on converging till they reach the second eye-glass D E: in consequence of passing through that second lens, their convergence is increased, and the rays enter the eye of the observer at P. I told you, when explaining the diagram of an eye looking at a cross, that the eye perceives only the appearance presented to it, and is unconscious of the refraction produced by the lens; regarding the object as if there were no lens at all, The converging lines PD, PE, are therefore prolonged in the same direction to S DIAGRAM EXPLAINED. 169 and T, where the magnified image appears to fill the whole space between those letters. If it were not for the refraction produced by the second eye-glass, the image would have been only the size of that which you see halfway between ST and AB, therefore, you perceive that the field of view, the space which can be commanded by the eye, is enlarged by the introduction of the second eye-glass. You know that all persons do not see objects clearly at the same precise distance; some are near-sighted, others can distinguish objects a great way off: in order to meet this diversity in the structure of different eyes, the body of the microscope is made to slide up and down in the cylinder, which lengthens or shortens the distance between the eye and the object examined, enabling the beholder to adjust the instrument so as to suit his own eye, and the focus of the object-glass he happens to use. From what has been said of the two instruments, you perceive that there is a great resemblance between telescopes and compound micro |