28 



THE ART OF MAKING MICROSCOPE SLIDES 



Algae 



fluid containing them considerable quanti- 

 ties of the required preservative, to permit 

 the algae to settle to the bottom, and then 

 to pour off the supernatant Uquid. One 

 of the modern plankton centrifuges will 

 do the job twice as efficiently in half the 

 time. These plankton centrifuges are 

 built as miniature milk separators save 

 for the fact that the vertical plates of the 

 latter are missing. That is, the plankton 

 centrifuge is merely a small cup which is 

 rotated at high speeds while a continuous 

 stream of the material to be concentrated 

 is poured into the top. The plankton 

 organisms, being the heavier, are collected 

 round the edge while the cleared water 

 passes out at the bottom. The material 

 should be put through the separator twice 

 and with its aid it is possible to con- 

 centrate five gallons of plankton into 

 100 milhliters in about five minutes. These 

 concentrates must be processed immedi- 

 ately; within a space of ten minutes the 

 available oxygen in the water will have 

 been used up and the concentrate will 

 die with a consequent degradation of its 

 appearance. 



The solution of Ripart and Petit, here 

 recommended, may be used as a fixative 

 for animal tissues as well as a preservative 

 for plant tissues and is accordingly given 

 in Chapter 18 under the heading F 

 4000.0010. fcit is a weak solution of 

 copper acetate and copper chloride, acid- 

 ified with acetic acid and with a small 

 quantity of camphor added. More modern 

 writers (Mayer 1920, p. 232) have sug- 

 gested the substitution of thymol for 

 camphor, and menthol may equally well 

 be employed. It is unwise to use a satu- 

 rated solution of any of these compounds, 

 for crystals are likely to form through the 

 slight evaporation which always takes 

 place in a mount. One therefore takes 

 equal quantities of a saturated solution of 

 camphor or thymol, and distilled water, 

 and then adds to each hter of this mixture 

 two grams each of copper acetate and 

 copper chloride together with seven milh- 

 liters of acetic acid. About ten times its 

 own volume of preservative should be 

 added to the concentrate, the bottle con- 

 taining which is then carefully tilted 

 backward and forward at intervals for the 



next twenty-four hours before the organ- 

 isms arc allowed finally to concentrate at 

 the bottom of the jar and the supernatant 

 reagent poured off. This preserv^ed con- 

 centrate may be kept indefinitely in the 

 dark and mounts made at any time. 



Preparation of the actual mounts must 

 be done on two successive days: on the 

 first of these the cells are prepared and 

 placed on one side to become hard; on 

 the second the actual mount is made. 

 Clean sHdes are absolutely necessary and 

 may either be cleaned in the manner sug- 

 gested in the last chapter, or may be 

 chemically cleaned by one of the cleaning 

 mixtures given in Chapter 28. The sKdes 

 should be selected rather more carefully 

 than usual for the most minute flaw in 

 the surface of the slide will become appar- 

 ent in an aqueous fluid mount, even 

 though it would be in\dsible in a colloidal 

 or resin medium of higher refractive 

 index. Having selected and cleaned the 

 shdes, a ring of gold size is turned on each, 

 care being taken that the ring is smaller in 

 diameter than is the coverslip to be em- 

 ployed. An 18-milhmeter ring and a 

 %-inch coverslip form an excellent com- 

 bination. It may be pointed out that this is 

 the reverse of what is done when mount- 

 ing in a tin, cardboard, or plastic cell 

 where a %-inch cell is used with an 

 18-milhmeter covershp. In this case the 

 purpose of the initial ring is not only to 

 provide support to the covershp but also 

 to insure that the cement subsequently 

 used for sealing shall not run in by capil- 

 lary attraction and ruin the mount. The 

 ring should be as narrow as can be drawn 

 and should be about }i4 of an inch thick 

 when in the fluid stain. With experience, 

 and a fine sable brush, it is possible to 

 draw these initial rings about ^i^ of an 

 inch in width, though Ke is permissible 

 and wiU be more hkely in the hands of the 

 inexperienced. As many rings are prepared 

 as mounts are to be made and placed on 

 one side until the next day. If, through 

 some accident, mounting cannot be con- 

 tinued on the next day, or possibly the 

 day after, it will be necessary to put a 

 thin coat of fresh gold size over the dry 

 coat and permit this to harden for 24 

 hours. The condition of the gold-size 



