1. **Problem Statement:** We are given a circular field of view with a diameter of 1.4 millimeters and a unicellular organism inside it. We need to determine the length of this unicellular organism. 2. **Understanding the Problem:** The diameter of the field of view is the maximum distance across the circle, which is 1.4 mm. The organism is inside this circle, so its length must be less than or equal to 1.4 mm. 3. **Assumption:** Since the organism is elongated and fits inside the circular field, the length of the organism is approximately the diameter of the field of view if it stretches across the circle. 4. **Conclusion:** Therefore, the length of the unicellular organism is approximately equal to the diameter of the field of view. 5. **Final Answer:** $$\text{Length of organism} = 1.4 \text{ mm}$$ This means the organism's length is 1.4 millimeters.