THE BRITISH JOURNAL OF OPHTHALMOLOGY types of natural intra-ocular colour-filters These various devices are quite strictly limited to vertebrate forms of similarly circumscribed visual habits. They are almost completely mutually exclusive; only one of these types is ordinarily present in a given animal or group, and none is found in any of the forms which possess oil-droplets. The difficulty of formulating a teleological interpretation of the oildroplet mosaic is immensely lessened by the consideration of the other, less well known and at first thought simpler types of intra-ocular filters. We believe'that as the facts are set forth, the first suspicion of the reader-that all of these apparently so dissimilar types of filters actually serve a common purpose-will become as firm a conviction in his mind as it is in ours. We have omitted a complete survey of the literature of this field; it is almost wholly devoted to the oil-droplets, and many of the contributions are rendered meaningless, from our standpoint, by a few critical researches to which we shall refer. But little descriptive work has been done upon the oil-droplets in recent years, and the reader will get an adequate conception of the oil-droplet as a structure from such works as Greeff (1900), Garten (1907), Hess (1912), Putter (1912), and Franz (1913). We shallcontent ourselves with a fair statement of each of the theories wbich have been advanced in explanation of the oil-droplet type of filter, and shall attempt to formulate an explanation in common terms of the entire assemblage of filter types.
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