The correct answer is $\boxed{\text{B) 0.4}}$.
The additive constant is the distance between the trunnion axis and the principal plane of the object glass. The principal plane is a virtual plane that divides the object glass
into two equal parts. The front principal plane is the plane that contains the optical center of the object glass and the back principal plane is the plane that contains the focal point of the object glass.The additive constant is calculated using the following formula:
$C = \frac{f}{2} – d$
where:
- $C$ is the additive constant
- $f$ is the focal length of the object glass
- $d$ is the distance from the object glass to the trunnion axis
In this case, the focal length of the object glass is 25 cm and the distance from the object glass to the trunnion axis is 15 cm. Therefore, the additive constant is:
$C = \frac{25}{2} – 15 = 0.4$
The additive constant is used to correct for the distance between the trunnion axis and the principal plane. This is necessary because the trunnion axis is not always located at the same position on the object glass. The additive constant is also used to correct for the thickness of the object glass.