METHODS OF STUDY 25 



quality material to take ordinary ink well, especially in damp 

 weather and the opening too large and too shallow to afford 

 safety to specimens of any size. Manufactured slides with 

 central opening and of considerable depth are now obtainable 

 and meet the requirements much better than most of the com- 

 mercial slides. 



Dr. Zinndorf of Germany has produced a slide which is very 

 excellent in many ways. It is made of black or dark blue cellu- 

 loid with a central cavity and a square glass cover held in place 

 by two clips cemented to the slide. These slides have the advant- 

 age of being very firmly closed yet easily opened, and specimens 

 can be kept loose in the slides with a large margin of safety. 

 The main objection to them is the danger of celluloid, and slides 

 of other substances such as bakelite can be made as cheaply. 



Another type of pasteboard slide that is used to good advan- 

 tage is one with a large rectangular opening, t"he background 

 made of double-thick photographic paper in black with white 

 lines forming 10, 20, 50, or 100 squares, each with a white num- 

 ber. When these are covered with a glass sheath, they make ex- 

 cellent slides for the rapid study of faunas. 



All the slides so far described are 3x1 inches. Dr. A. Franke 

 of Arnstadt has advocated the use of a smaller slide which is 

 much more compact. These are described in his papers. They 

 are arranged in narrow trays which may be placed under the 

 microscope and all the slides of a tray examined one by one 

 without removing the slide from the tray. Dr. Franke showed 

 me his collection mounted in these slides and they certainly have 

 the advantage of taking up very little space. 



Specimens may be kept loose in the openings of slides that are 

 covered with the glass sheaths or in the Zinndorf type of slide. 

 With most slides and for greater safety, specimens should be 

 attached. For this purpose, gum of some sort is necessary. 

 Ordinary glue will crack on drying and frequently break the 

 specimen. It is difficult to soften glue when it is desired to 

 change the position of a specimen. In this laboratory the gum 

 that has been found most satisfactory is one made by dissolving 

 ordinary gum arable in warm water to form a consistency de- 

 sirable for the size specimens to be mounted, then adding enough 

 glycerin to prevent cracking. A drop or two of formaldehyde 

 can be added to avoid moulding. This gum is transparent and 



