ultramicroscopic
adjective
ul·tra·mi·cro·scop·ic ˌəl-trə-ˌmī-krə-ˈskä-pik 
variants or less commonly ultramicroscopical
ˌəl-trə-ˌmī-krə-ˈskä-pi-kəl
1
: too small to be seen with an ordinary microscope
2
: of or relating to an ultramicroscope
ultramicroscopically
ˌəl-trə-ˌmī-krə-ˈskä-pi-k(ə-)lē
adverb 
Word History
Etymology
International Scientific Vocabulary
First Known Use
1870, in the meaning defined at sense 1
Medical Definition
ultramicroscopic
adjective
ul·tra·mi·cro·scop·ic -ˌmī-krə-ˈskäp-ik
variants also ultramicroscopical
-i-kəl
1
: too small to be seen with an ordinary microscope compare macroscopic, microscopic sense 2, submicroscopic
2
: of or relating to an ultramicroscope