PRACTICAL PHOTO-MICROGKAPHY. 37 



in facility and elegance, is a simple adapter with a bayonet 

 joint made by filing away part of the thread of the adapter, 

 and part of that of the microscope tube. A lens can be 

 screwed into the latter in the usual way just as if the tube- 

 thread were complete. 



AH* 



FIG. 8. TRIPLE NOSE-PIECE. 



The objective screws into one end of the adapter in the usual 

 way, then the other end of the adapter is pushed straight into 

 the threaded end of the microscope tube and gets a short turn 

 in one direction, when it is at once clamped. These adapters 

 are easily made, small, cheap, and several can be had for a few 

 shillings. 



No mention has been yet made of oculars, because the 

 writer has no confidence in any ocular except those made 

 specially for projection, and projection oculars shall be treated 

 in a separate chapter along with apochromatic objectives, It 

 is not by any means asserted that no ocular of the ordinary 

 type may be successfully used in photo-micrography, but the 

 writer has never yet been satisfied with the performance in 

 this work of any ocular, achromatic or otherwise, except the 

 projection oculars. It is not denied that in some cases good 

 results have been obtained with ordinary eye-pieces, but the 

 writer has never yet ascertained on what fortuitous circum- 

 stances the success has depended. " Fortuitous " is written 

 advisedly, for by its construction an ordinary ocular is evi- 

 dently not intended to project an image, except on the human 

 retina. It is to be noted that an ordinary objective is " over- 

 corrected," a Huyghenian ocular under-corrected, so the two 

 often balance each other. 



