270 American Qiiarto-ly Microscopical Journal. 



half long, slips easily. The slip-tube (4) is provided at one end 

 with the Society's screw, by which a three inch objective (5), or 

 any other preferred for the purpose, can be attached. The mova- 

 ble square plate is provided with a spring catch (6) which fits 

 into any one of a series of notches in the edge of a brass quadrant 

 (7), and thus serves both to hold the plate in position and to 

 register the angle of obliquity. The transverse bar (i) slips in 

 a groove on the upper surface of a strong brass tube (8) fitted 

 to the substage of the microscope. The bar itself has a longi- 

 tudinal slot running nearly its whole length, so that it can be 

 pushed to any desired position without disturbing the position of 

 a central steel rod (10) at the upper end of which a lens (9) is 

 fastened. The lens (9) is such a segment of a hemisphere of 

 crown-glass that, when brought into optical contact (by oil of 

 cloves) with the under surface of an ordinary glass object-slip, 

 the object to be studied will be as nearly as possible at its center 

 of curvature ; and the rod (10) slips freely in the top of the sub- 

 stage tube (8), so that the lens may be pushed into position or 

 withdrawn at pleasure. 



In using this apparatus with monochromatic sunlight, 1 first set 

 the square brass-plate (3) at the desired angle, as read on the 

 quadrant, and then slip the transverse bar (i) backwards or for- 

 wards, as may be necessary, until the pencil of monochromatic 

 sunlight (to which the desired degree of obliquity has been pre- 

 viously given by means of a prism) falls centrally through the 

 slip-tube (4) and illuminating objective (5) upon the face of the 

 lens with which the object is viewed. By means of the slij)- 

 tube, the illuminating objective (5) is then brought to the proper 

 focal position. Ordinary illumination is thus obtained, of any 

 desired obliquity from about 30" to the limit of the thickness of 

 the stage. When I desire still greater obliquity, I use Powell and 

 Lealand's extra stage, and slij) the transverse bar into the groove 

 at the upper end of the holder which those makers provide with it 

 to carry the small bull's-eye they furnish for the examination of 

 Amphipleiira pellucida. In this manner I can get more oblique 

 illumination up to 80" or even 85°; but, of course, the oblique 

 pencils thus obtained are refracted at the under surface of the 

 glass slip that carries the object, and cannot possibly reach the 

 object itself at an oblicjuity greater than 41". To obtain greater 

 obliquity than this, I make use of the hemispherical lens (9). The 

 illuminating objective is set at the desired angle, say 45", and the 



