Whoever carries a mental kodak with him (as I suspect I was in the habit of doing long before I knew it) must be aware of the uncertain value of the different exposures. This can be determined only by the process of developing, which requires a dark room and other apparatus not always at hand; and so much depends upon the process that it might be well if it could always be left to some one who makes a specialty of it, as in the case of the real amateur photographer.