1. The problem asks to express the number $2.6 \times 10^{-4}$ in ordinary decimal form. 2. The scientific notation $a \times 10^{n}$ means move the decimal point in $a$ by $n$ places. 3. Since the exponent is $-4$, move the decimal point 4 places to the left. 4. Starting with $2.6$, moving the decimal 4 places left adds zeros: $0.00026$. 5. Therefore, $2.6 \times 10^{-4}$ in ordinary form is $0.00026$. This is the final answer.