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Bausch & Lomb Optical Company The projection lenses supplied on the Model C Balopticon are of the new Balo series, the highest grade lenses made for lantern slide projection. They are furnished in two diameters: 1^4 and 2 %g inches. The former is quite satisfactory when used with the arc lamp, since the light source is small compared to that of the Mazda lamp or acetylene burner. The 2%6-inch diameter lens should be used for all focal lengths over 10 inches and by all means with such illuminants as the Mazda lamp and acetylene burner. In the latter cases the larger size lens gives an increase of about 40 per cent, in illumination. Double Dissolving Outfits Figure 4—Double Dissolving Model C Balopticon. To avoid the eyestrain and disagreeable effect produced by the alternate darken- ing and lighting of the screen, together with the “racing” of the slides across the screen in changing, so-called dissolving outfits are employed, by means of which each view is made to fade gradually and blend into the succeeding picture. This effect is very satisfactorily accomplished by combining two Model C Balopticons and fitting the lenses with our patent iris diaphragm dissolver. By means of simple connections the lanterns are mounted one above the other, so that the two fields are coincident upon the screen. The iris diaphragm dissolver consists of an iris diaphragm fitted to each lens, the two being jointly operated by a connecting rod which opens one diaphragm as the light is shut off by the other. The two Balopticons can be quickly put together or dismounted, so that the lanterns can be used separately in class rooms and combined for use in the auditorium. 29