56 THE AMERICAN MONTHLY [March, 



the substage ring so that it may be accurately centred, which is essential 

 to its best performance, and so that it may be moved upward to bring 

 the light from the mirror to a focus on the object, or removed from the 

 latter toward the mirror to reduce the intensity of the illumination. 

 Some method, therefore, for changing its postition vertically, either by 

 rack and pinion or by direct finger movements, is absolutely essential. 



It is always accompanied by diaphragms to reduce the size of the 

 illuminating cone, to obtain light of great obliquity, or black-ground 

 illumination. In the best condensers these are applied below the lenses, 

 while in the cheaper and less desirable forms they are placed above the 

 uppermost lens. 



Low power objectives, those, for instance, up to the one-four or 

 one-fifth inch, and those of small angular aperture, do not call for 

 the use of the condenser. Sufficient illumination, generally more than 

 is needed, may be had from the concave mirror alone. For small-angled 

 lenses, if a condenser is desired, the one-inch objective, or an eye-piece 

 of the proper construction, is useful. The objective, when used for 

 this purpose, is screwed into an adapter fitting the substage ring with 

 the front lens upward, the light being reflected from the plane mirror. 

 If the Acme or other microscopical lamp be used, or if a bull's-eye 

 condensing lens be interposed between the light and the mirror, the. 

 objective is then to be raised or lowered until the proper illumination is 

 obtained. With this, however, only central light may be used. 



Messrs. J. W. Queen and Co. make a simple and useful form, with 

 an adapter for the substage ring, centring adjustments, and three dia- 

 phragms, which are placed above the lenses. The diaphragm with the 

 smallest opening is used here, as elsewhere, for centring the condenser 

 to the objective, the former being moved from side to side and forward 

 or backward, while the eye is at the top of the body tube, the ocular 

 having been removed. This small opening can be easily seen through 

 the objective as a bright spot of light which must be brought accurately 

 to the centre. For low powers where a larger field is to be illuminated 

 with less intensity than with high powers, the upper lens of this con- 

 denser is removed, and the lower one focussed on the object in the usual 

 way after the diaphragm with the largest opening has been applied. 

 This is a commendable, inexpensive condenser for use with small an- 

 gled, dry objectives, but for the best wide-angled objectives there are 

 other forms better adapted to the purpose. 



Several English and American opticians have produced substage 

 condensers which are praiseworthy in some respects, but it was for 

 Prof. Abbe to devise the best ever offered by any optician. This is the 

 popular Abbe condenser, as supplied by Zeiss, of Jena, and so fre- 

 quently referred to in microscopical literature. 



The condenser as made by Zeiss for his own stands is very large and 

 heavy, and is not intended to be used on those of any other maker. 

 He says that since this is the case ''adaptation to stands of other make, 

 therefore, is nearly always impracticable and will not be undertaken." 

 The contrivance is too useful, however, to be abandoned by English 

 and American microscopists, so that the majority of our opticians make 

 modified forms, which preserve the essential features and are adapted 

 to American and English stands. Mr. Zentmayer, Mr. Grunow, 

 Messrs. J. W. Queen & Co., and Messrs. Bausch & Lomb all offer 



