THE AMERICAN MONTHLY 



[January, 



nearer to it than to tj-Z^^, and nearer than 

 is commonly beheved. I beheve the cor- 

 rect average will be found to be below 

 Tj-Jjj^ and about gJ^^ of an inch. 



Second: I do not think my words, if 

 carefully read, will support the inference 

 that objectives and apparatus of thirty 

 years ago "were so imperfect that trust- 

 worthy results could not be obtained by 

 them." What I say, on page 225, that a 

 comparison made by each microscopist for 

 hiinself can be relied on by him, applies 

 as well to older as to later measurements. 

 The point of my objection is that while the 

 older work may have been accurate, we 

 have no sufficient proof to justify us in 

 treating it as absolutely so ; and when we 

 compare our measurements with the older 

 ones and deduce a result from the coinci- 

 dence, or lack of it, we are treating the 

 older measurement as absolutely correct. 



Third : That the older measurements, 

 even if made with inferior objectives and 

 erroneous micrometers, may indicate cor- 

 rectly relative sizes, I fully concede, and 

 it is clearly set forth in my previous com- 

 munication. Indeed, correct ruling of the 

 micrometer as to value of spaces is quite 

 unnecessary for a correct comparison of 

 relative sizes; but it is otherwise if the 

 spacing is uneven, and this was the very 

 respect in which the older micrometers 

 were most frequently defective. But the 

 comparison of recent measurements with 

 the old ones to determine a result is treat- 

 ing the older results as absolute sizes in- 

 stead of relative ones. This I object to. 



Fourth : An objective of inferior correc- 

 tions may give an increased size to the 

 image of the object. With the best pres- 

 ent objectives observers will often differ as 

 to the exact focus, and with the older objec- 

 tives there was even more room for doubt. 

 In many of the old measurements powers 

 of 800, 600, 500, and even lower were used. 

 The error due to coma would be propor- 

 tionately greater in these measurements 

 than in those with equally imperfect ob- 

 jectives of higher power. If in using a 

 power of 1 500 an object ^^^ of an inch in 

 diameter measures 20 divisions of an eye- 

 piece micrometer to one observer and 21 

 to another, the difference is nearly 5 per 

 cent. I have known this to occur, yet the 

 absolute value of the one space would be 

 but 6OTOT of ^^ iJ^ch. C. M. VoRCE. 



To THE Editor. — In the Microscopical 

 Journal just received, I notice your re- 

 marks as to the expulsion of air from 

 microscopic objects by the use of boiled 

 water. Allow me to suggest that when 



desiring to wash and mount objects con- 

 taining a considerable quantity of air, you 

 try the following plan : Take, for exam- 

 ple, a collection of Isthmia, or some other 

 diatom. The valves enclose so much 

 air as to cause them to float upon water, 

 and it must be extracted, for until they 

 sink it is impossible to wash them. Drive 

 from water all the air you can by a good 

 boiling for about five minutes, allow the 

 water to cool so as to be in condition to 

 absorb air, and without delay drop in the 

 diatoms. The water will extract the air 

 from them and they will go to the bottom. 

 Then add to the water a little dissolved 

 chloride of soda, and, with an occasional 

 shake up, you will find the material pretty 

 well cleaned and bleached in one hour. 

 Wash thoroughly in several changes of 

 water. 



Take a drachm of redistilled alcohol and 

 add thereto two drops of dissolved gum 

 Arabic, such as is obtained at an artist's 

 furnishing store. With a sharpened stick 

 place a small quantity on the centre of a 

 cleaned slide. It will spread out and the 

 alcohol will quickly evaporate, leaving a 

 very thin film of the gum. On this gummed 

 spot place a drop of your cleaned diatoms 

 and see that they are thoroughly dried by 

 time or heat. Of course, they are now 

 filled with air and are firmly enough at- 

 tached to the slide, and can be covered 

 in a cell if a dry mount is desired. 



To movmt in balsam, however, the air 

 must be again extracted, and at this stage 

 the boiled water prescription cannot be ad- 

 ministered. Have Canada balsam made 

 quite tough by age or heat and then dis- 

 solved in benzole ; and have some pure 

 benzole. Put around the objects which 

 have been dried on the slide a few frag- 

 ments of cover-glass, and on them, as legs 

 to a stool, place a clean cover-glass. A 

 drop of the pure benzole will quickly run 

 under the cover-glass and very promptly 

 take the place of the air in the diatoms ; 

 and a drop of the balsam at one edge of 

 the cover and a corner of blotting-paper 

 at the other will quickly substitute the bal- 

 sam for the benzole. Time or gentle heat 

 will harden the cement and the specimen 

 is safe. 



By the way, unless the object to be 

 mounted is very thick, it is unnecessary 

 to construct a cell. Fragments of cover- 

 glass will save from crushing. 



D. S. W. 



" The Science of Fibrine." 

 To THE Editor. — I have not infre- 

 quently seen in microscopical and medical 



