Why increase the amount of light projected through the specimen under an oil immersion objective?

Answer 1

Are you asking why does the amount of light increase once you view a specimen under oil imersion or why do you increase the amount of light entering the specimen when you are using oil imersion.

I can answer the first one - This happens because the density of the glass of the slide and the oil is the same therefore there is no refraction between the 2. Where normally there would be refraction between the glass slide and the air and then from the air to the objective. And in the latter case some light would be lost. Therefore with oil imersion all the light enters the objective and the clearest image is seen at high power.

As to why you would use more light under oil. The only reason I can think of is if you have a very dense specimen and you are viewing at very high power the light may be blocked by the small part of the specimen being veiwed. Therefore if you increase the incident light a greater concentration may be able to transfuse the specimen.