190 



THE AMERICAN MONTHLY 



[October, 



make a plus correction of one degree 

 (centigrade) for observations in which 

 the temperature of tlie air of the room 

 and of the microscope fell below his 

 normal (20° c.) as far as 12 to 15°. 

 Practically, in use these observations 

 are consequently made almost alto- 

 gether on large cavities and under 

 low-power objectives, and an accu- 

 i-acy to one degree centigrade has been 

 accepted as satisfactory. Although 

 wide discrepancies have constantly 

 occurred, even in determinations on 

 the fluid cavities in the same slice of 

 mineral by means of these devices, 

 on the other hand some of the most 

 delicate and important investigations, 

 such as those of Sorby and King on 

 the indication of the degrees of pres- 

 sure to which certain granytes have 

 been subjected during folding and 

 metamorphism, have rested largely 

 upon the accuracy of determinations 

 of this very kind. 



In my previous paper I have de- 

 scribed two forms of apparatus found- 

 ed on the plan of immersion of both 

 the objective and the thin section un- 

 der examination in baths which may 

 be warmed sufficiently by the direct 

 application of the flame of a taper, or 

 by conduction through a copper plate, 

 or by the slow addition of water of a 

 different temperature, or, in a warm 

 room, by a current of the observer's 

 breath . All intei-ference with the inves- 

 tigation , ei ther by warm or cool atmos - 

 pheric currents, or by radiation into 

 or from adjacent metal, is eliminated 

 by this plan. I afterwards found that 

 the suggestion of the idea of immer- 

 sion of the thin section alone had al- 

 ready occurred to Brewster, Vogel- 

 sang and Sorby, who had in occasional 

 single investigations simply immersed 

 the mineral in heated water or glyce- 

 rin, stiring up the liquid with a 

 thermometer. The range of error 

 found in this partial application of 

 the idea amounted to 13° c, even 

 with low-power objectives ; the pro- 

 cess was found tedious and slow, and 

 Vogelsang preferred the use of his 

 galvanic apparatus. 



In place of the two forms of appa- 

 ratus described in my former paper, 

 I now present a more finished arrange- 

 ment, consisting of a thin-walled box 

 heated by conduction from a taper 

 through the copper plate which forms 

 its bottom and projects beyond the 

 stage. The thermometer, made to 

 order by the well known instrument 

 maker to the Smithsonian — Mr. Henry 

 Green, of New- York — has a scale 

 ranging little beyond 22-45° c, each 

 degree on the scale being two cm. in 

 length and divided into tenths. The 

 bore and length of the tube is so ar- 

 ranged as to bring that part of the 

 scale near 30° c. on a level with the 

 eye, w^hile stationed at the eye-piece, 

 to facilitate an instantaneous observa- 

 tion, by a glance, of the height of the 

 mercurial column, at any moment, 

 without a removal or even movement 

 of the head. I find as before that not 

 only the high-power immersion objec- 

 tives, such as the ^^g of Spencer or -^ 

 of Powell and Lealand, can be used in 

 this way, but that apparently any low- 

 power objectives so far tried have 

 their fittings water-tight, and can be 

 used while half immersed in the bath, 

 only occasionally with a little loss of 

 definition or brightness, sometimes 

 with a decided improvement in both 

 qualities and with a complete dispen- 

 sation of the necessity for repeated 

 refocussing otherwise requii'ed. The 

 observation of the critical point in the 

 fluid, checked by both the tempera- 

 tures of disappearance and reappear- 

 ance of the bubble, may be easily ef- 

 fected in most cases within ten min- 

 utes and to an accuracy of at least 

 one-twentieth of a degree centigrade. 

 For various further particulars and 

 precautions, I need but refer to my 

 former paper. 



On Some Points in Microtomy.* 



BY JOHN A. RYDER, EMBRYOLOGIST 

 TO THE U. S. FISH COMMISSION. 



The attention of investigators has 

 been so greatly occupied with this 



♦Abstract of paper announced for Section G, A. A. 

 A. S., but not read. 



