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Bausch & Lomb Optical Comp any those used on automobiles and motorcycles for headlight illumination. These can be rented or exchanged at almost any automobile supply store. This illuminant is very popular among institute workers and others lecturing in rural districts, where electricity is not often available. The oxyhydrogen burner gives illumination exceeding in Oxyhydrogen brilliancy that of the acetylene burner, but is not so convenient Burner ' . , , and costs somewhat more to operate. An oxyhydrogen generator is recommended as a source of supply. Projection Lenses Selection of Lenses Improvements on the optical parts of our Balopticons have been in keeping with the mechanical development, as evidenced by our new series of Balo projection lenses. These superior objectives are of an entirely new type of con- struction, particularly well corrected for flatness of field and critical definiton. They are made in three series as described under “Accessories” (see index). These new lenses are now supplied on all but two of our Balopticon models. In purchasing an apparatus for the projection of either lan- tern slides or opaque objects a point which should receive careful attention is the selection of an outfit with the correct focal length of lens to meet the specific requirements satisfactorily. The focal length of the projection lens, all other conditions being equal, determines the size of picture on the screen. In selecting an apparatus, therefore, one should determine the distance at which the lantern is to be placed from the screen and the size of picture desired. With these two factors given, the focal length of lens required can readily be determined by reference to one of the tables here given. We would particularly caution Balopticon users not to make the mistake, too often made when working with a projection lantern, of deciding upon too large a picture. Any picture out of proportion in size to the distance at which it is being viewed appears unnatural and lacking in perspective and is a strain to the eyes of the observer. Another point to bear in mind is that the intensity of illumination per unit of area varies inversely as the square of the width of the picture; therefore, the smaller the picture the more brilliant it will be. On the other hand, the picture should be large enough, of course, to enable those seated at the farthest points from the screen to see all of its details without difficulty. A fairly safe rule to follow, according to Prof. S. H. Gage, of Cornell University, is to determine upon a picture equal in width to about % or Ys the distance from the screen to the farthest point at which it will be observed. Having thus established the two determining factors men- tioned, one has only to refer to that table below which covers the kind of projection and the size of projected area offered by his apparatus. The focal lengths here tabulated are those which are listed with some one of our different Balopticons for either lantern slides or opaque objects. The Projection Tables 17