A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 199 



evenly-lighted field. In these experiments it will be advisable 

 to use some simple form of light modifier, such as a piece of 

 signal-green glass, and a rather high eyepiece may be employed 

 with advantage in order to render differences in the (juality of 

 the various images more prominently visible. It should also bfr 

 borne in mind, and experimentally verified, that when the back 

 lens of the objective is nearly filled with light, it is extremely 

 sensitive to any alteration of tube length or cover-glass thickness,, 

 a very slight increase or decrease of the former being sufficient 

 to make or mar the sharpness of the resultant picture, while any 

 oversight in making this adjustment will inevitably cause it to 

 appear veiled and milky, even with a first-class apochromatic 

 objective. Indeed, the better the lens employed, the more 

 sensitive it will be found in this respect. If the beginner will 

 only examine a number of miscellaneous objects under the above 

 specified conditions, there can be but little doubt that his con- 

 version to critical microscopy, even for low-power every-day 

 purposes, will be speedy. 



Passing on to the second cause of the avoidance of critical 

 methods of research by general observers, especially such as 

 habitually employ low amplifications — i.e., their strong predi- 

 lection for an entirely illuminated "full-moon" field — it should 

 be realised that if this condition be considered a sine qua non, 

 there are several ways of easily obtaining it without putting 

 the sub-stage condenser out of focus ; that method will not be 

 thought desirable by any one who has tried the experiments 

 recommended above. Neither is the expedient of placing an 

 auxiliary condenser in front of the lamp fiame to be altogether- 

 commended, although a properly arranged and focussed aplanatic 

 " bull's-eye " thus used is, in skilful hands, capable of affording 

 good (but not strictly critical) results when compared to the 

 images obtainable with an out-of-focus sub-stage condenser. 

 The proper and easy way to obtain perfectly critical representa- 

 tions of objects viewed under a l|-in. or 1-in. objective, with 

 the whole field fully illuminated, is to employ a very low 

 power achromatic condenser, affording a sufficiently large 

 aplanatic cone for the purpose. This maximum aperture need 

 not exceed "3 for the 1-in., and may be, of course, considerably- 

 less for the Ig-'n. ; but as it will be frequently found 

 advantageous to utilise such an appliance with a |-in.,. 



