For these reasons it will be of importance to the student of microscopy to begin at the beginning, and it will be my endeavour to introduce to his notice such facts in physical optics as are closely associated with the formation of images, and, so to speak, systematise such stepping stones for work hereafter to be accomplished. Elementary principles only will be adduced, and without attempting to involve my readers in intricate mathematical problems, and which for the most part are unnecessary for the attainment of the object in view. I therefore pass at once to the consideration of the propagation of light through certain bodies.
The microscope, whether simple or compound, depends for its magnifying power on the influence exerted by lenses in altering the course of the rays of light passing through them being REFRACTED. _Refraction_ takes place in accordance with two well-known laws of optics. When a ray of light passes from one transparent medium to another it undergoes a change of direction at the surface of separation, so that its course in the second medium makes an angle with its course in the first. This change of direction is a resultant of refraction. The broken appearance presented by a stick partly immersed in water, and viewed in an oblique position, is an illustration of the law of refraction. Liquids have a greater refractive power than air or gases. As a rule, with some few exceptions, the denser of the two substances has the greater refractive power; hence it is customary in enumerating some of the laws of optics to speak of the denser medium and the rarer medium. The more correct designation would be the more refractive and the less refractive.[5]
[Illustration: Fig. 4.--Law of Refraction.]
Let R I (Fig. 4) be a ray incident at I on the surface of separation of two media, and let I S′ be the course of the ray after refraction. Then the angles which R I and I S make with the normal are the _angle of incidence_ and the _angle of refraction_ respectively, and the first law of refraction is that these angles lie in the same plane, or the _plane of refraction_ is the same as the _plane of incidence_. The law which connects the magnitudes of these angles, and which was discovered by Snell, a Dutch philosopher, can only be stated either by reference to a geometrical construction, or by using the language of trigonometry. Describe a circle about the point of incidence, I as a centre, and drop perpendiculars from the points where it cuts the rays on the normal. The law is that these perpendiculars, R′ P′, S′ P, will have a constant ratio, or the sines _of the angles of incidence and refraction are in a constant ratio_; that is, so long as the media through which the ray first passes, and by which it is afterwards refracted, remain the same, and the light also of the same kind, then it is referred to as the law of sines.