THE OBJECT 103 



because focusing up and down does not disturb the correc- 

 tions, as it does in both dry and oil-immersion objectives. 



3. Forty-five per cent acetic acid causes the chromosomes 

 and nuclei to stand out distinctly in the transparent 

 cytoplasm. Saturated with carmine, with a trace of iron, 

 it serves as a fixative, stain, and mounting medium, all 

 in one. For rapid fixation of chromosomes, few reagents 

 approach it. It has an index of refraction only a little 

 above that of water, and suits water-immersion objectives. 



4. Paraffin oil (thick) was used by Coles without a 

 cover-glass. He put it on dry preparations of bacteria 

 or spirochsetes mixed with granular matter, when viewing 

 them, without a cover, with a dry objective on a dark 

 field (46). A thin layer only was used. They then showed 

 up clearly. 



The refraction of ordinary paraffin oil is less than that 

 of thickened cedar oil. In the writer's experience, ordinary 

 paraffin oil cannot be substituted for the special immersion 

 cedar oil without badly degrading the performance of the 

 objectives (see also Coles, 46). 



5. According to Britton (''Flora of the Northern States 

 and Canada"), Canada balsam comes from Ahies halsamea 

 (L.) Mill., the balsam fir. Either in the natural state, or 

 dried and dissolved in xylol, it has long been used to mount 

 sections or smear preparations, in which the parts to be 

 studied, if naturally colorless, have previously been well 

 stained. Since neither the refraction nor the dispersion 

 of the mixture of balsam and xylol are constant, nor equal 

 to those of the immersion oil above the cover, when oil- 

 immersion objectives are used, objects for accurate study 

 should be mounted in immersion oil. Canada balsam is 

 often acid, and causes stains to fade (Bolles Lee, 

 Vademecum). 



6. Cedar oil comes presumably from the pencil cedar, 

 Juniperus virginiana L. The thin oil is thickened by 

 exposure to the air till it has the index of 1.515 for the D 

 line, the original oil having the index 1.510. It continues 

 thickening and rising in refractive index, on further 



