EXPLANATION OF PLATE LI. (continued). 



FIG. 



7, 8. Illustrating the arrangement of the microscope and lamp employed by the author 

 for obtaining the most satisfactory illumination and definition of minute 

 flagellate organisms, when working with object-glasses of i-i6th to i-5oth 

 inch nominal focal distance, for which he is chiefly indebted to a most kind 

 and painstaking demonstration by Mr. E. M. Nelson, F.R.M.S. The mirror 

 m being turned to one side, the microscope and lamp are so disposed that 

 the central ray of light ax from the narrow edge of the lamp flame passes 

 through the optical axis of the achromatic condenser a c, and is then focussed 

 upon the field of view, by means of the substage rackwork, in such a manner 

 that employing a i-inch object-glass, a sharply defined image of the lamp- 

 flame, edge on, is projected upon the centre of the field in company with the 

 objects under examination as shown at Fig. 8. If the i-inch object-glass is 

 now detached, and a i-i6th, i-25th or i-5oth substituted, and focussed into 

 place, a slight readjustment of the centering of the achromatic condenser 

 being perhaps required, it will be found that the entire field is brilliantly 

 illuminated, and the most minute objects defined with an amount of sharp- 

 ness rarely obtained under other conditions. In addition to the ordinary 

 graduating diaphragm placed immediately beneath the lenses of the achro- 

 matic condenser as at d 1 in Fig. 7, the author has derived considerable 

 advantage from the interposition of a second diaphragm at d z , or the lowest 

 point in the substage arrangement. 



9, 10. Trichocysts of Bursaria (Panophrys) leucas (see vol. i. p. 82), as interpreted by 

 Professor G. J. Allman ; Fig. 9, trichocysts, tr, in sitti, disposed in an even 

 vertical layer immediately beneath the cuticle and locomotive cilia c, x loco. 

 Fig. 10, the same trichocysts projected irregularly from the entire peri- 

 phery as hairlike filaments or setae with recurved distal ends, on the appli- 

 cation of acetic acid or forcible compression (after Allman, ' Quarterly 

 Journal of Microscopical Science,' vol. iii., 1855). 



