332 NATURAL HISTORY OF OUR SHORES 



frequently, and examine, so as not to rub away the slice 

 altogether, but it must be brought to transparency, not 

 thicker than tissue paper. 



Now place the bit of glass and its adherent section in a 

 saucer with turpentine. The balsam will dissolve,- and the 

 section will be free. Wash it well in turpentine, and see 

 that no emery grains are on it. Then transfer to the centre 

 of a glass slip ; put on a drop of balsam and benzole ; cover, 

 clip, and finish as in previous work. 



A section of the great Pinna shell is a good type for 

 illustration of shell structure. 



This method of grinding down and mounting applies to 

 all hard substances bones, teeth, and stones. (A flat iron 

 plate is used for the last instead of ground glass.) 



Mounting entire Objects in Balsam., etc. We have now 

 gone over what is usually considered the difficult portion of 

 microscopic manipulation. The mounting of objects in 

 their entirety is a simple matter. The proceeds of the 

 tow net (page 299) furnishes abundant material the young 

 of fishes, crustaceans, the little pelagic annelids, etc. 



These objects for balsam mounts usually require staining. 

 This is done by giving them a rather prolonged immersion 

 in a dilute stain. If a strong one is used the outside parts 

 will be too deeply stained while the internal structure, 

 which it is desired to show, is not stained at all. In all 

 cases it is well to remove some of the superficial stain. 

 This is done by placing the objects in dilute spirit, to which 

 a trace the very slightest perceptible of nitric acid has 

 been added. (For Calcareous objects this must, however, not 

 be employed.) Watch the progress of decoloration, and 

 check it at the desired point, by rapidly transferring the 

 objects to pure spirit. Transfer to absolute alcohol, and 

 let them dehydrate thoroughly ; clear well in oil of cloves, 

 and mount, without pressure, in a sunk cell of such depth 

 that the cover glass does not bear on the object. If the 



