and Laboratory Methods. -'iT9 



moduli of the various combinations are once determined and recorded for refer- 

 ence the stage micrometer will not be needed thereafter except for verification. 



The mode of procedure is as follows : Place the eyepiece micrometer in 

 place on the diaphragm of the ocular. The stage micrometer is also placed in 

 position upon the stage of the microscope as an ordinary slide and the instru- 

 ment focused upon it until the scale is in distinct view. The two scales are then 

 seen simultaneously, and one superimposed upon the other. 



For the sake of illustration suppose 20 of the divisions on the stage micro- 

 meter are found to coincide with 12 of those on the eyepiece micrometer. If 

 the former is ruled in hundredths of a millimeter (10 micromillimeters) then for 

 this particular combination of objective and ocular each scale division of the eye- 

 piece micrometer equals .20 mm. divided by 12, which gives .0166 mm.; or as 

 commonly expressed in microscopic work 16.6 ja. The Greek letter //, called 

 mu, is the symbol used for one micromillimeter or micron (.001 mm.). 



In making measurements with this combination thereafter the object to be 

 measured is placed upon a slide upon the stage of the microscope as usual and 

 the size determined in units of the divisions of the eyepiece micrometer. To 

 reduce this reading to microns it must be multiplied by the micrometer modulus, 

 which in the above supposed case was found to be 16.6. 



Another method of making microscopic measurements is described by Ives^ 

 which is ingenious as well as novel. In this method the image of a jeweller's 

 saw or some similar object placed in direct line with the source of light is 

 brought into the focal plane of the objective by focusing the substage condenser 

 and using the plane mirror. This method requires the use of a stage micro- 

 meter to obtain the micrometer modulus, but removes the necessity of the eye- 

 piece micrometer. Though not as convenient as the method first described, 

 good results can nevertheless be obtained if proper precautions are taken to 

 always have the distances uniform. 



For more complete directions concerning the use of the microscope and its 

 accessories many books are to be had. Among these might be mentioned the 

 following: "The Microscope," by Gage ; " Manipulation of the Microscope," 

 by Bausch ; and "Practical Methods in Microscopy," by Clark. 

 Bureau of Chemistry, U. S. Dept, of Agriculture. BuRTON J. HoWARD, 



To Clean Homogeneous Immersion Objectives. 



On page 2298 of the Journal Mr. C. M, Clark mentions " the water " to 

 replace benzine, xylene, etc, in cleaning homogeneous immersion objectives. I 

 can confirm the statement of Mr, Clark entirely while adding to it a modification. 



After having used an homogeneous immersion objective, I first clean the 

 objective and the slide with a piece of old dry linen of fine texture, then I moisten 

 an end of the linen with a little saliva and after having gently rubbed the objec- 

 tive front therewith I ascertain if the cleaning is perfect, using a magnifier in 

 the examination. 



Thanks to the slight quantity of soda contained in the saliva the cleaning is 

 perfect and practically instantaneous. 



I have used homogeneous immersion objectives since 1878, when they were 

 introduced, and never, either for those used with cedar oil or for those with 

 glycerine immerson (ToUes and Spencer) or for those of NA 1.60 with mono- 

 bromonaphthalin, have I employed any other method of cleaning. The saliva 

 cleanses so well that the front of my objectives are still as clear as when new. 

 Director of Botanical Garden of Antwerp (Belgium). Dr. Henri Van Hewick. 



1 Ives, " Making Measurements in the Microscope," Journal Franklin Institute, July, 1902, 

 P- 73- 



