i86 American Quarterly Microscopical Journal. 



ON THE FORMATION OF THE PARABOLOID AS AN 

 ILLUMINATOR FOR THE MICROSCOPE. 



BY F. H. WENHAM. 



{Received Dece^nber 21 si, iSyS.) 



In a paper read before the Microscopical Society, in the year 

 1S56, I announced a new and important principle of illuminat- 

 ing objects under the highest powers of the microscope, or 

 with close working object glasses of large aperture, utilizing 

 the glass cover as a lieberkiihn by various methods of causing 

 rays to impinge upon the upper surface beyond the angle of 

 total reflection. The principle was described in the following 

 sentence: "The principle of operation consists in causing rays 

 of light to pass through the under side of the glass slip upon 

 which the object is mounted, at the proper angle for causing 

 total internal reflection from the upper surface of the thin cover, whicli 

 is thus made to act the part of a speculum for throwing light 

 down upon the under-lying objects immersed in the balsam or 

 fluid." 



The first and most simple method was by means of a right- 

 angled prism connected to the under surface of the slide by a 

 fluid intermedium, which transferred the total reflecting sur- 

 face from the prism on to the top plane of the cover. The next 

 illustration referred to a truncated lens considered as a hemi- 

 sphere minus the thickness of the slide. In this I demonstrated 

 the loss of light from using water as the intermedium, and the 

 advantage of introducing a connecting fluid of a refractive in- 

 dex the same as the glass, in order to utilize the most oblique 

 rays which would otherwise not be transmitted;* for this pur- 

 pose I selected oil of cloves. In the paper I showed a form of 

 truncated paraboloid mounted so as to be used also as an ani- 

 malcule cage or live box. the object to be placed in water on 

 the flat top and confined by a thin glass cover: rays from a lamp 

 made parallel to be sent in beneath, using a dry object glass. 



* For the purpose of eliminating the aberrations in an immersion object glass, caused by an in- 

 termedium of low refractive power, in the Monthly MitRoscopiCAL Journal for June 1870 1 

 suggested the "oil immersion" object glass in the following words: " If a medium of similar 

 refraction to the glass were to be used, no adjustment would be required for any thickness of 

 cover, supposing the test objects to be mounted thereon (they generally are), for, in fact, we should 

 then view them all with a front of the same thickness — considering the cover, the front len*-, and 

 th^ interposing medium as one." 



