Plate VI.[40] represents the microscopic appearance of genuine butter and oleomargarine. It will be observed that in butter (Fig. 1) numerous globules but no crystals of fat are presented, the crystals present being those of salt. In oleomargarine (Fig. 2) the distinctive pear-shaped masses of fat, accompanied by only a small number of fatty globules, are to be seen. Dr. Thomas Taylor (of the U.S. Department of Agriculture), has made an elaborate investigation of the microscopic appearance of various fats when viewed by polarised light. He regards the presence of peculiar globular crystals and the black cross commonly termed St. Andrew’s cross as characteristic of genuine butter.[41] Lard, beef, and other fats are said to exhibit different and, to some extent, distinctive crystalline forms. Prof. Weber,[42] however, affirms that mixtures of lard and tallow fat, under certain conditions, cannot be distinguished from butter by means of this method of examination. More recently, Dr. Taylor states that the distinguishing difference between butter and other fats under the microscope is that the former, when observed by polarised light through a selenite, exhibits a uniform tint, whereas the latter shows prismatic colours. Although the results of these investigations cannot as yet be considered as perfectly satisfactory or conclusive, they certainly are entitled to rank as a highly valuable and important step in advance of the optical processes hitherto employed. PLATE VI. [Illustration: Fig. 1, Butter × 400.] [Illustration: Fig. 2, Oleomargarine × 400.] ARTOTYPE. E. BIERSTADT, N. Y. PLATE VII. [Illustration: Butter × 40.] [Illustration: Butter × 40.] [Illustration: Beef × 40.] [Illustration: Lard × 160.] [Illustration: Butterine × 40.] [Illustration: Oleomargarine × 40.] FAT CRYSTALS. Plate VII. exhibits the appearance of butter, oleomargarine, beef, and some other fats, when viewed by the microscope and polarised light. It will be noticed that, while a discrimination between lard and butter is readily made, oleomargarine presents the St. Andrew’s cross, stated to be characteristic of genuine butter. These photomicrographs represent the results of investigations made in the Chemical Division of the U.S. Department of Agriculture. The question of the sanitary effects of oleomargarine and other substitutes for butter, has been studied by many scientists, and with very discordant results. Doubtless the great divergence of opinion which at present exists, is largely due to the fact that the artificial products examined have been made according to different processes, and with varying regard to the quality of the fats used in their manufacture, and to the degree of care and cleanliness observed. The attention of the American public has very lately been directed to the oleomargarine question, by the recent enactment of a national law imposing a tax upon the manufacture of the article.