After performing Koehler illumination, what adjustment should be made to ensure optimal contrast?

To achieve optimal contrast after performing Koehler illumination, adjusting the condenser diaphragm by closing it slightly is essential. This action reduces the amount of light entering the condenser, which helps to enhance the contrast between the specimen and the background. When the diaphragm is opened fully, excessive light can wash out details in the specimen, making it harder to distinguish between different structures or features.

By closing the diaphragm by about one-quarter to one-third, the light is more focused and can be better modulated for the specific characteristics of the sample being observed. This adjustment facilitates the detection of fine details and improves the overall quality of the microscopic image.

Using a higher magnification objective or changing the light intensity can affect image clarity and brightness but may not specifically enhance contrast as effectively as adjusting the diaphragm. The diaphragm's role in controlling light that reaches the specimen is crucial for achieving the desired quality in the visual representation of the sample.