Finally, in order to fully understand the consequences of the sign convention for the signs of the various paraxial quantities defining the image forming process, it is a valuable exercise to consider the paraxial ray tracing formulae. These are discussed in detail by Welford [2.3] and various formulations are possible. As stated above, the equations are fundamentally linear and, in the general case for following the path of a ray through successive refracting or reflecting surfaces, consist of a refraction equation and a transfer equation to the next surface. Here, we follow the old procedure with individual steps, because the signs of intermediate quantities are of fundamental importance in understanding the numerous relationships of paraxial quantities in the basic telescope forms discussed in § 2.2.5 below. The complete paraxial ray trace for refraction (or reflection) at a surface and transfer to the next is then written: a) Refraction at surface v lv =
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