50 BULLETIN OF THE LABORATORIES 



fogged negatives and light struck plates and with a glass cutter cut 

 them into lantern size and then utilize the opaque film by scratching 

 in designs with a sharp point. These are especially useiul for pre- 

 sentation of formulae and tables and outline work. 



A sec<'nd class of slides may be made by transfere, a sort of de- 

 calcimania process, which as a rule has not been found satisfactory, 

 though sometimes employed. The picture desired may occur in a 

 newspaper, it can be cut out and cemented face down on the glass 

 with some cement insoluble in water, (collodian or negative varnish,) 

 then the paper can be moistened and by careful rubbing with the fin- 

 gers can all be removed and leave the ink of the picture on the glass. 

 This should also be varnished to protect it. 



The third class of slides may be made by the use of thin objects 

 and sections mounted between glasses, opaque objects whose outline 

 is desired. Skeleton leaves to show venation. Histological prepara- 

 tions which are large enough. Of course it is not intended to include 

 under this head preparations for microscopic projection, as that would 

 make a subject for separate consideration. 



We would now speak of the slides whicli are made by photo- 

 graphic methods. 



And here the simplest slide is the negative. A negative is the 

 first plate or film made with a camera or otherwise, which reverses the 

 lights and shades. Very many slides show to a betcer advantage in 

 the negative than in the positive. A drawing may be made with a 

 black crayon on a white paper and with a white crayon on a black- 

 board — both may be positives. Many book illustrations which it is 

 desired to copy into a. slide hav^e a black background. Their negative 

 shows a white background and a dark object. The use of a negative 

 from a book illustration which had a white back ground, in many 

 cases, is very satisfactory. A sure rule is: Whenever the background 

 or the shade of the design does not represent a recognized character 

 of the object presented a negalive is equally as appropriate as a positive 

 When this is the case the shade should always accord with the recog- 

 nized character. For example, one may choose to illustrate the reces- 

 sion of the Falls of St. Anthony, from the plates in the Minnesota 

 Report. The water must be represented as white, hence a negative 

 must be made for each illustration and from them a positive, requir- 

 ing two plates, two developments and two elements of time for the 

 production of each slide. While going through just that tedious task 

 the method which is about to be described was discovered. When it 

 is desired to obtain but one slide from a photograph, picture or illus- 



