294 King Tiridates was converted by Gregory the Illuminator and adopted the Christian faith.
The objects observed with the vertical illuminator must not have a glass cover if the dry system is employed, because the upper surface of the glass cover would send so much light back into the objective by reflection, that the image would be indistinct.
Gregory the Illuminator (Rivingtons, 1868).
It is best if the image of the light is not larger than the object examined, and to effect this, an illuminating lens with an iris diaphragm is often placed between the source of light and the illuminator.
It is, on the contrary, possible to examine covered objects with the vertical illuminator, if the immersion system be employed.