Oil Immersion Microscopy?

[Brad S]

After focusing under x400, a student attempts to focus a culture of E. coli using the x1000 objective and oil immersion. The student was unable to find the culture. Can you suggest what may have gone wrong?

The question is very broad and general, I can't think of a diffinitive answer. I have thought of three possible answers:

(i) The student hadn't found and focused on a colony whilst using the x400 lens before switching to the more powerful x1000 (or he moved the slide out of place).

(ii) She has used the coarse-adjustment knob instead of the fine-adjustment knob while looking under the x1000.

(iii) The slide is upside-down.

I think if more imformation was given in the question about how the slide was prepared and what actions the student committed before the use of the x1000 lens and oil immersion, there might be a more diffinitive answer. What are your suggestions?