AND SANDARAC IN MICROSCOPY. 15> 



Add the castor oil and the amylic alcohol to the sandarac in 

 a corked wide-mouthed bottle. Allow to stand, with occasional 

 shaking, until solution is complete. Filter through cotton wool. 



As some workers may prefer to weigh the castor oil rather 

 than measure it, a suitable alternative formula is given. For 

 this mounting medium I have proposed the name amyl-sandarac 



The refractive index of the medium is a little difficult to 

 determine because the amylic alcohol is the ingredient of lowest 

 index, viz. 140, and is constantly evaporating whilst the 

 medium is drying. The result is that the mountant in a finished 

 preparation has an index depending upon the extent to which 

 the alcohol has disappeared. The final condition of the medium 

 will also vary according as the mount is made in the ordinary 

 way or by the exposure method. In the former case experiment 

 shows that the refractive index is about 145 while in the latter 

 it is about 148. 



The amylic alcohol is the constituent that is mainly respon- 

 sible for the low refractive index, and if one substitutes, in the 

 above formula, the resin of Canada balsam for sandarac one 

 obtains a very similar medium — amyl-balsam — which can be 

 used in the same way as the amyl-sandarac. 



For various reasons the sandarac medium is preferable. In 

 the first place, sandarac does not require any preliminary heating 

 to remove non-resinous volatile constituents as is the case with 

 Canada balsam, but is suitable for use in its original state. 

 Secondly, the pale colour of sandarac is so marked that the 

 medium appears quite colourless in the case of all ordinary 

 mounts, and in the case of thick mounts, such as are necessary 

 for whole insects and other bulky objects, the mountant possesses 

 only a very pale lemon-yellow tint in place of the deep golden 

 yellow of similar balsam mounts.* 



* The effect of amyl-sandarac and amyl-balsam in renderings 

 visible certain delicate structures owing to the low refractive index 

 of these media was well shown in photographs taken from prepara- 

 tions of crystals of calcium carbonate obtained as a scale on evaporat- 

 ing a well-water. The delicate rosette crystals so apparently abun- 

 dant in the amyl-sandarac preparation were hardly visible and, if 

 not known to be present, would probably be overlooked in the benzol- 

 balsam mount. 



