( 17 ) 



Adding' less hydrofluoric acid, the process does not go to the end. As 

 it is necessary to cover the slide by means of Canada, balsam (Soi.las) 

 in order to preserve the object glass from the disastrous influence 

 of the vapours of hydrofluoric acid, it is difticult to vary the acidity 

 of the fluid, in which the spicules are mounted. Also it is impossible 

 in this way, to exclude the influence of the glass. We had therefore, 

 to construct an apparatus allowing the concentration of the hydrofluoric 

 acid to be varied without danger to the lenses of the microscope. 

 At first we tried ebonite, in combination with glass, covered by a 

 layer of celluloid, which is sold in solution under the name of zapon. 

 But as this method did not satisfy our purposes, we tried another 

 and we think that we have found a good and rather simple device. 

 The adjoined figure needs little explanation. Out of a sheet of trans- 



parent celluloid, 1 mm. thick, is constructed a case, abed ') ; the 

 bottom measures 6 X ^^ <^'™-' the height is 1.5 cm. In the midst 

 of the case a circular rim of celluloid, high 5 mm,, is joined to 

 the bottom. Anotlier case of celluloid a'b'c'd' measures 4.5 X 6 cm. 

 bottom and 8 mm. height. In the middle of the bottom a square of 

 2X^.5 cm. is cut out. This opennig is covered by a sheet of thin cel- 

 luloid, no more than 0.5 mm. thick, measuring 2.5 X 3.5 cm. This 

 thin sheet A is joined with b'c' air- and watertight by means of soft 

 paraffin. The spicules under examination are i)laced on the bottom 

 of the case a'b'c'd', e. g. in a drop of water. To flatten this drop 

 it is covered by a very thin film of celluloid. In the exterior part 

 of the case abed is poured out some liquid |)araffin p, in the interior 

 commercial hydrofluoric acid (//), diluted witJi 3 or 4 parts of water. 

 Now the case a'b'c'd' is reversed and put into abed. As we have 

 found that hydrofluoric acid in the gaseous state after some time 



1) The carefully cut and cleaned sheets of celkiloid are easily united by means 

 of acelone. The parts that should be united are pressed together, and by means 

 of a small brush a lillle drop of acetone is applied. Tlie celluloid immediately 

 sticks together. The corners are afterwards, to diminish the chance of leakage, 

 cemented another time with zapon. Celluloid is got at the D. Celluloid Fabrik 

 Leipzig. 



1) Very thin fdms of celluloid are got by pouring out zapon on a glassplate, 

 in the way that collodion plates are made, and tearing it off the glass after drying. 



Proceedings Royal Acad. Amsterdam. Vol. Vlil. 



